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Vector Calculus - Solutions 4th Edition

Susan Jane Colley

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INSTRUCTOR’S SOLUTIONS

MANUAL

SUSAN JANE COLLEY

DANIEL H. STEINBERG

V ECTOR C ALCULUS

FOURTH EDITION

Susan Jane Colley

Oberlin College

The author and publisher of this book have used their best efforts in preparing this book. These efforts include the development, research, and testing of the theories and programs to determine their effectiveness. The author and publisher make no warranty of any kind, expressed or implied, with regard to these programs or the documentation contained in this book. The author and publisher shall not be liable in any event for incidental or consequential damages in connection with, or arising out of, the furnishing, performance, or use of these programs.

Reproduced by Pearson from electronic files supplied by the author.

Copyright © 2012, 2006, 2002 Pearson Education, Inc.

Publishing as Pearson, 75 Arlington Street, Boston, MA 02116.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Printed in the United States of America.

ISBN-13: 978-0-321-78066-9

ISBN-10: 0-321-78066-3

www.pearsonhighered.com

Table of Contents

Chapter 1 Vectors

1.1Vectors in Two and Three Dimensions

1.2More About Vectors

1.3The Dot Product

1.4The Cross Product

1.5Equations for Planes; Distance Problems

1.6Some n-dimensional Geometry

1.7New Coordinate Systems

Exercises for Chapter 1

Exercises for Chapter 1

Chapter 2 Differentiation in Several Variables

2.1Functions of Several Variables; Graphing Surfaces

2.2Limits

2.3The Derivative

2.4Properties; Higher-order Partial Derivatives

2.5The Chain Rule

2.6Directional Derivatives

2.7Newton’s Method

Exercises for Chapter 2

for Chapter 2

Chapter 3 Vector-Valued Functions

3.1Parametrized Curves and Kepler’s Laws

3.2Arclength and Differential Geometry

3.3Vector Fields: An Introduction

3.4Gradient, Divergence, Curl and the Del Operator

Exercises for Chapter 3

Exercises for Chapter 3

Chapter 4 Maxima and Minima in Several Variables

4.1Differentials and Taylor’s Theorem

4.2Extrema of Functions

4.3Lagrange Multipliers

4.4Some Applications of Extrema

Exercises for Chapter 4

Exercises for Chapter 4

Chapter 5 Multiple Integration

5.1Introduction:

5.2Double

5.3Changing

5.4Triple

5.5Change of

5.6Applications

5.7Numerical

Chapter 6 Line Integrals

6.1Scalar

6.2Green’s

6.3Conservative

Chapter 7 Surface Integrals and Vector Analysis

7.1Parametrized

7.2Surface

7.3Stokes’s

7.4Further

Chapter 8 Vector Analysis in Higher Dimensions

8.1An

8.2Manifolds

8.3The

Chapter1

Vectors

1.1VectorsinTwoandThreeDimensions

1. Herewejustconnectthepoint(0,0)tothepointsindicated :

2. Althoughmoredifficultforstudentstorepresentthisonpaper,the figuresshouldlooksomethinglikethefollowing.Notethat theoriginisnotatacorneroftheframeboxbutisatthetailsofthethreevectors

Inproblems3and4,wesupplymoredetailthanisnecessarytostresstostudentswhatpropertiesarebeingused :

3.(a) (3, 1)+( 1, 7)=(3+[ 1], 1+7)=(2, 8)

(b) 2(8, 12)=( 2 8, 2 12)=( 16, 24).

(c) (8, 9)+3( 1, 2)=(8+3( 1), 9+3(2))=(5, 15)

(d) (1, 1)+5(2, 6) 3(10, 2)=(1+5 2 3 10, 1+5 6 3 2)=( 19, 25)

(e) (8, 10)+3((8, 2) 2(4, 5))=(8+3(8 2 4), 10+3( 2 2 5))=(8, 26)

4.(a) (2, 1, 2)+( 3, 9, 7)=(2 3, 1+9, 2+7)=( 1, 10, 9)

(b) 1 2 (8, 4, 1)+2 5, 7, 1 4 = 4, 2, 1 2 + 10, 14, 1 2 =(14, 12, 1)

(c) 2 (2, 0, 1) 6 1 2 , 4, 1 = 2((2, 0, 1) (3, 24, 6))= 2( 1, 24, 5)=(2, 48, 10)

5. Westartwiththetwovectors a and b.Wecancompletetheparallelogramasinthe figureontheleft.Thevectorfromthe origintothisnewvertexisthevector a + b.Inthe figureontherightwehavetranslatedvector b sothatitstailistheheadof vector a.Thesum a + b isthedirectedthirdsideofthistriangle.

6. a =(3, 2) b =( 1, 1) a b =(3 ( 1), 2 1)=(4, 1)

, 4)

7.(a) → AB =( 3 1, 3 0, 1 2)=( 4, 3, 1) → BA

AB =(4, 3, 1) (b) → AC =(2 1, 1 0, 5 2)=(1, 1, 3) → BC =(2 ( 3), 1 3, 5 1)=(5, 2, 4) → AC + → CB =(1, 1, 3) (5, 2, 4)=( 4, 3, 1) (c) Thisresultistrueingeneral:

Head-to-tailadditiondemonstratesthis.

8. Thevectors a =(1, 2, 1), b =(0, 2, 3) and a + b =(1, 2, 1)+(0, 2, 3)=(1, 0, 4) aregraphedbelow. Againnotethat theoriginisatthetailsofthevectorsinthe figure

Also, 1(1, 2, 1)=( 1, 2, 1).Thiswouldbepicturedbydrawingthevector(1,2,1)intheoppositedirection. Finally, 4(1, 2, 1)=(4, 8, 4) whichisfourtimesvector a andsoisvector a stretchedfourtimesaslonginthesamedirection.

9. Sincethesumontheleftmustequalthevectorontherightcomponentwise: 12+ x =2, 9+7= y ,and z + 3=5.Therefore, x =14, y =16,and z =8

10. Ifwedropaperpendicularfrom(3,1)tothe x-axisweseethatbythePythagoreanTheoremthelengthofthevector (3, 1)= √32 +12 = √10

11. Noticethat b (representedbythedottedline) =5a (representedbythesolidline).

12. Herethepicturehasbeenprojectedintotwodimensionssothatyoucanmoreclearlyseethat a (representedbythesolid line) = 2b (representedbythedottedline).

13. Thenaturalextensiontohigherdimensionsisthatwestilladdcomponentwiseandthatmultiplyingascalarbyavectormeans thatwemultiplyeachcomponentofthevectorbythescalar.Insymbolsthismeansthat: a + b =(a1 ,a2 ,...,an )+(b1 ,b2 ,...,bn )=(a1 + b1 ,a2 + b2 ,...,an + bn ) and k a =(ka1 ,ka2 ,...,kan )

Inourparticularexamples, (1, 2, 3, 4)+(5, 1, 2, 0)=(6, 1, 5, 4),and 2(7, 6, 3, 1)=(14, 12, 6, 2)

14. Thediagramsforparts(a),(b)and(c)aresimilartoFigure1.12fromthetext.Thedisplacementvectorsare:

(a) (1,1,5)

(b) ( 1, 2, 3)

(c) (1, 2, 3)

(d) ( 1, 2)

Note:Thedisplacementvectorsfor(b)and(c)arethesamebutinoppositedirections(i.e.,oneisthenegativeofthe other).Thedisplacementvectorinthediagramfor(d)isrepresentedbythesolidlineinthe figurebelow:

15. Ingeneral,wewoulddefinethedisplacementvectorfrom (a1 ,a2 ,...,an ) to (b1 ,b2 ,...,bn ) tobe (b1 a1 ,b2 a2 ,...,bn an )

Inthisspecificproblemthedisplacementvectorfrom P1 to P2 is (1, 4, 1, 1).

16. Let B havecoordinates(x, y , z ).Then −→ AB =(x 2,y 5,z +6)=(12, 3, 7) so x =14, y =2, z =1 so B has coordinates(14,2,1).

17. If a isyourdisplacementvectorfromtheEmpireStateBuildingand b yourfriend’s,thenthedisplacementvectorfromyou toyourfriendis b a.

18. Property2followsimmediatelyfromtheassociativepropertyofthereals:

Property3alsofollowsfromthecorrespondingcomponentwiseobservation:

19. Weprovidetheproofsfor R3 : (1)(k + l )a =(k + l )(a1 ,a2 ,a3 )=((k + l )a1 , (k + l )a2 , (k + l )a3 ) =(ka1 + la1 ,ka2 + la2 ,ka3 + la3 )= k a + l a (2) k (a + b)= k ((a1 ,a2 ,a3 )+(b1 ,b2 ,b3 ))= k (a1 + b1 ,a2 + b2 ,a3 + b3 ) =(k (a1 + b1 ),k (a2 + b2 ),k (a3 + b3 ))=(ka1 + kb1 ,ka2 + kb2 ,ka3 + kb3 ) =(ka1 ,ka2 ,ka3 )+(kb1 ,kb2 ,kb3 )= k a + k b (3) k (l a)= k (l (a1 ,a2 ,a3 ))= k (la1 ,la2 ,la3 ) =(kla1 ,kla2 ,kla3 )=(lka1 ,lka2 ,lka3 ) = l (ka1 ,ka2 ,ka3 )= l (k a)

20.(a) 0a isthezerovector.Forexample,in R3 :

(b) 1a = a.Againin R3 : 1

21.(a) Theheadofthevector sa isonthe x-axisbetween0and2.Similarlytheheadofthevector tb liessomewhereonthe vector b.Usingthehead-to-tailmethod, sa + tb istheresultoftranslatingthevector tb,inthiscase,totherightby2s (representedinthe figureby tb*).Theresultisclearlyinsidetheparallelogramdeterminedby a and b (andisonlyonthe boundaryoftheparallelogramifeither t or s is0or1.

(b) Againthevectors a and b willdetermineaparallelogram(withverticesattheorigin,andattheheadsof a, b,and a + b Thevectors sa + tb willbethepositionvectorsforallpointsinthatparallelogramdeterminedby(2,2,1)and(0,3,2).

22. HerewearetranslatingthesituationinExercise21bythevector → OP0 .Thevectorswillallbeoftheform → OP0 + sa + tb for 0 ≤ s,t ≤ 1

23.(a) Thespeedofthe fleaisthelengthofthevelocityvector = ( 1)2 +( 2)2 = √5 unitsperminute.

(b) After3minutesthe fleaisat (3, 2)+3( 1, 2)=(0, 4)

(c) Wesolve (3, 2)+ t( 1, 2)=( 4, 12) for t andgetthat t =7 minutes.Notethat both 3 7= 4 and 2 14= 12

(d) Wecanseethisalgebraicallyorgeometrically:Solvingthe x partof (3, 2)+ t( 1, 2)=( 13, 27) wegetthat t =16.Butwhen t =16, y = 30 not 27.Alsointhe figurebelowweseethepathtakenbythe fleawillmissthe point ( 13, 27).

24.(a) Theplaneisclimbingatarateof4milesperhour.

(b) Tomakesurethattheaxesareorientedsothattheplanepassesoverthebuilding,thepositive x directioniseastandthe positive y directionisnorth.Thenweareheadingeastatarateof50milesperhouratthesametimewe’reheadingnorth atarateof100milesperhour.Wearedirectlyovertheskyscraperin1/10ofanhouror6minutes.

(c) Usingouranswerin(b),wehavetraveledfor1/10ofanhourandsowe’veclimbed4/10ofamileor2112feet.Theplane is 2112 1250 or862feetabouttheskyscraper.

25.(a) Addingweget: F1 + F2 =(2, 7, 1)+(3, 2, 5)=(5, 5, 4)

(b) Youneedaforceofthesamemagnitudeintheoppositedirection,so F3 = (5, 5, 4)=( 5, 5, 4)

26.(a) Measuringtheforceinpoundsweget (0, 0, 50)

(b) The z componentsofthetwovectorsalongtheropesmustbeequalandtheirsummustbeoppositeofthe z component inpart(a).Their y componentsmustalsobeoppositeeachother.Sincethevectorpointsinthedirection(0, ±2,1), the y componentwillbetwicethe z component.Togetherthismeansthatthevectorinthedirectionof (0, 2, 1) is (0, 50, 25) andthevectorinthedirection(0,2,1)is(0,50,25).

27. Theforce F duetogravityontheweightisgivenby F =(0, 0, 10).Theforcesalongtheropesareeachparalleltothe displacementvectorsfromtheweighttotherespectiveanchorpoints.Thatis,thetensionvectorsalongtheropesare

F1 = k ((3, 0, 4) (1, 2, 3))= k (2, 2, 1)

F2 = l ((0, 3, 5) (1, 2, 3))= l ( 1, 1, 2), where k and l areappropriatescalars.Fortheweighttoremaininequilibrium,wemusthave F1 + F2 + F = 0,or,equivalently, that k (2, 2, 1)+ l ( 1, 1, 2)+(0, 0, 10)=(0, 0, 0). Takingcomponents,weobtainasystemofthreeequations:

+2l =10

Solving,we findthat k =2and l =4,sothat F1 =(4, 4, 2) and F2 =( 4, 4, 8)

1.2MoreaboutVectors

ItmaybeusefultopointoutthattheanswerstoExercises1and5arethe“same”,butthatinExercise1, i =(1, 0) andinExercise 5, i =(1, 0, 0).ThiscomesupwhengoingtheotherdirectioninExercises9and10.Inotherwords,it’snotalwaysclearwhether theexercise“lives”in R2 or R3

1. (2, 4)=2(1, 0)+4(0, 1)=2i +4j

2. (9, 6)=9(1, 0) 6(0, 1)=9i 6j

3. (3,π, 7)=3(1, 0, 0)+ π (0, 1, 0) 7(0, 0, 1)=3i + π j 7k.

4. ( 1, 2, 5)= 1(1, 0, 0)+2(0, 1, 0)+5(0, 0, 1)= i +2j +5k.

5. (2, 4, 0)=2(1, 0, 0)+4(0, 1, 0)=2i +4j.

6. i + j 3k =(1, 0, 0)+(0, 1, 0) 3(0, 0, 1)=(1, 1, 3)

7. 9i 2j + √2k =9(1, 0, 0) 2(0, 1, 0)+ √2(0, 0, 1)=(9, 2, √2)

8. 3(2i 7k)= 6i +21k = 6(1, 0, 0)+21(0, 0, 1)=( 6, 0, 21)

9. π i j = π (1, 0) (0, 1)=(π, 1)

10. π i j = π (1, 0, 0) (0, 1, 0)=(π, 1, 0)

Note:YoumaywanttoassignbothExercises11and12togethersothatthestudentsmayseethedifference.Youshouldstress thatthereasontheresultsaredifferenthasnothingtodowiththefactthatExercise11isaquestionabout R2 whileExercise12is aquestionabout R3

11.(a) (3, 1)= c1 (1, 1)+ c2 (1, 1)=(c1 + c2 ,c1 c2 ),so c1 + c2 =3, and c1 c2 =1

Solvingsimultaneously(forinstancebyaddingthetwoequations),we findthat 2c1 =4,so c1 =2 and c2 =1.So b =2a1 + a2 (b) Here c1 + c2 =3 and c1 c2 = 5,so c1 = 1 and c2 =4.So b = a1 +4a2 (c) Moregenerally, (b1 ,b2 )=(c1 + c2 ,c1 c2 ),so c1 + c2 = b1 , and c1 c2 = b2

Againsolvingsimultaneously, c

12. Notethat a3 = a1 + a2 ,soreallyweareonlyworkingwithtwo(linearlyindependent)vectors. (a) (5, 6, 5)= c1 (1, 0, 1)+ c2 (0, 1, 0)+ c3 (1, 1, 1);thisgivesustheequations:

The firstandlastequationscontainthesameinformationandsowehaveinfinitelymanysolutions.Youwillquicklysee onebyletting c3 =0.Then c1 =5 and c2 =6.Sowecouldwrite b =5a1 +6a2 .Moregenerally,youcanchooseany valuefor c1 andthenlet c2 = c1 +1 and c3 =5 c1 (b) Wecannotwrite(2,3,4)asalinearcombinationof a1 , a2 ,and a3 .Herewegettheequations:

c1 + c3 =2 c2 + c3 =3 c1 c3 =4

The firstandlastequationsareinconsistentandsothesystemcannotbesolved. (c) Aswesawinpart(b),notallvectorsin R3 canbewrittenintermsof a1 , a2 ,and a3 .Infact,onlyvectorsoftheform (a,b, a) canbewrittenintermsof a1 , a2 ,and a3 .Foryourstudentswhohavehadlinearalgebra,thisisbecausethe vectors a1 , a2 ,and a3 arenotlinearlyindependent.

Note:Aspointedoutinthetext,theanswersfor13–21arenotunique.

13. r(t)=(2, 1, 5)+ t(1, 3, 6) so ⎧ ⎨ ⎩ x =2+ t y = 1+3t z =5 6t.

14. r(t)=(12, 2, 0)+ t(5, 12, 1) so ⎧ ⎨ ⎩ x =12+5t y = 2 12t z = t.

15. r(t)=(2, 1)+ t(1, 7) so x =2+ t y = 1 7t.

16. r(t)=(2, 1, 2)+ t(3 2, 1 1, 5 2) so ⎧ ⎨ ⎩ x =2+ t y =1 2t z =2+3t.

17. r(t)=(1, 4, 5)+ t(2 1, 4 4, 1 5) so ⎧ ⎨ ⎩ x =1+ t y =4 z =5 6t.

18. r(t)=(8, 5)+ t(1 8, 7 5) so x =8 7t y =5+2t.

Note:Inhigherdimensions,weswitchournotationto xi

19. r(t)=(1, 2, 0, 4)+ t( 2, 5, 3, 7) so ⎧ ⎪ ⎪ ⎨ ⎪ ⎪ ⎩ x1 =1 2t x2 =2+5t x3 =3t x4 =4+7t.

20. r(t)=(9,π, 1, 5, 2)+ t( 1 9, 1 π, √2+1, 7 5, 1 2) so

21.(a) r(t)=( 1, 7, 3)+ t(2, 1, 5) so ⎧ ⎨ ⎩ x = 1+2t y =7 t z =3+5t.

(b) r(t)=(5, 3, 4)+ t(0 5, 1+3, 9 4) so ⎧

x =5 5t y = 3+4t z =4+5t.

x1 =9 10t x2 = π +(1 π )t x3 = 1+(√2+1)t x4 =5+2t x5 =2 t.

(c) Ofcourse,thereareinfinitelymanysolutions.Forourvariationontheanswerto(a)wenotethatalineparalleltothe vector 2i j +5k isalsoparalleltothevector (2i j +5k) soanothersetofequationsforpart(a)is:

x = 1 2t y =7+ t z =3 5t.

Forourvariationontheanswerto(b)wenotethatthelinepassesthroughbothpointssowecansetuptheequationwith respecttotheotherpoint:

x = 5t y =1+4t z =9+5t.

(d) Thesymmetricformsare: x +1 2 =7 y = z 3 5 (for(a)) 5 x 5 = y +3 4 = z 4 5 (for(b)) x +1 2 = y 7= z 3 5 (forthevariationof(a)) x 5 = y 1 4 = z 9 5 (forthevariationof(b))

22. Solvefor t ineachoftheparametricequations.Thus

andthesymmetricformis

23. Solvingfor t ineachoftheparametricequationsgives t = x 7, t =(y +9)/3,and t =(z 6)/( 8),sothatthesymmetric formis

24. Seteachpieceoftheequationequalto t andsolve:

25. Let t =(x +5)/3.Then x =3t 5.Inviewofthesymmetricform,wealsohavethat t =(y 1)/7 and t =(z +10)/( 2) Henceasetofparametricequationsis x =3t 5, y =7t +1,and z = 2

Note:InExercises26–29,wecouldsayforcertainthattwolinesarenotthesameifthevectorswerenotmultiplesofeach other.Inotherwords,ittakestwopiecesofinformationtospecifyaline.Youeitherneedtwopoints,orapointandadirection(or inthecaseof R2 ,equivalently,aslope)

26. The firstlineisparalleltothevector a1 =(5, 3, 4),whilethesecondisparallelto a2 =(10, 5, 8).Since a1 and a2 are notparallel,thelinescannotbethesame.

27. Ifwemultiplyeachofthepiecesinthesecondsymmetricformby 2,weareeffectivelyjusttraversingthesamepathata differentspeedandwiththeoppositeorientation.Sothesecondsetofequationsbecomes:

Thislooksalotmorelikethe firstsetofequations.Ifwenowsubtractonefromeachpieceofthesecondsetofequations(as suggestedinthetext),weareeffectivelyjustchangingourinitialpointbutwearestillonthesameline:

Wehavetransformedthesecondsetofequationsintothe firstandthereforeseethattheybothrepresentthesamelinein R3 .

28. Ifyou firstwritetheequationofthetwolinesinvectorform,wecanseeimmediatelythattheirdirectionvectorsarethesame soeithertheyareparallelortheyarethesameline:

r1 (t)=( 5, 2, 1)+ t(2, 3, 6) r2 (t)=(1, 11, 17) t(2, 3, 6).

The firstlinecontainsthepoint ( 5, 2, 1).Ifthesecondlinecontains ( 5, 2, 1),thentheequationsrepresentthesameline. Solvejustthe x componenttogetthat 5=1 2t ⇒ t =3.Checkingweseethat r2 (3)=(1, 11, 17) 3(2, 3, 6)= ( 5, 2, 1) sothelinesarethesame.

29. Hereagainthevectorformsofthetwolinescanbewrittensothatweseetheirheadingsarethesame:

r1 (t)=(2, 7, 1)+ t(3, 1, 5)

r2 (t)=( 1, 8, 3)+2t(3, 1, 5)

Thepoint (2, 7, 1) isonlineone,sowewillchecktoseeifitisalsoonlinetwo.AsinExercise28wechecktheequationfor the x componentandseethat 1+6t =2 ⇒ t =1/2.Checkingweseethat r2 (1/2)=( 1, 8, 3)+(1/2)(2)(3, 1, 5)= (2, 7, 2) =(2, 7, 1) sotheequationsdonotrepresentthesamelines.

Note:ItisagoodideatoassignbothExercises30and31together.Althoughtheylooksimilar,thereisadifferencethat studentsmightmiss.

30. Ifyoumakethesubstitution u = t3 ,theequationsbecome:

x =3u +7, y = u +2, and z =5u +1

Themap u = t3 isabijection.Theimportantfactisthat u takesonexactlythesamevaluesthat t does,justatdifferenttimes. Since u takesonallreals,theparametricequationsdodeterminealine(it’sjustthatthespeedalongthelineisnotconstant).

31. Thistimeifyoumakethesubstitution u = t2 ,theequationsbecome:

x =5u 1, y =2u +3, and z = u +1

Theproblemisthat u cannottakeonnegativevaluessotheseparametricequationsareforaraywithendpoint ( 1, 3, 1) and heading (5, 2, 1).

32.(a) Thevectorformoftheequationsis: r(t)=(7, 2, 1)+ t(2, 1, 3).Theinitialpointisthen r(0)=(7, 2, 1),andafter 3minutesthebirdisat r(3)=(7, 2, 1)+3(2, 1, 3)=(13, 1, 8)

(b) (2, 1, 3)

(c) Weonlyneedtocheckonecomponent(saythe x): 7+2t =34/3 ⇒ t =13/6.Checkingweseethat r 13 6 = (7, 2, 1)+ 13 6 (2, 1, 3)= 34 3 , 1 6 , 11 2

(d) Asinpart(c),we’llcheckthe x componentandseethat 7+2t =17 when t =5.Wethenchecktoseethat r(5)= (7, 2, 1)+5(2, 1, 3)=(17, 3, 14) =(17, 4, 14) so,no,thebirddoesn’treach (17, 4, 14)

33. Wecansubstitutetheparametricformsof x, y ,and z intotheequationfortheplaneandsolvefor t.So (3t 5)+3(2 t) (6t)=19 whichgivesus t = 3.Substitutingbackintheparametricequations,we findthatthepointofintersectionis ( 14, 5, 18)

34. UsingthesametechniqueasinExercise33, 5(1 4t) 2(t 3/2)+(2t +1)=1 whichsimplifiesto t =2/5.Thismeans thepointofintersectionis ( 3/5, 11/10, 9/5)

35. Wewillseteachofthecoordinateequationsequaltozerointurnandsubstitutethatvalueof t intotheothertwoequations.

x =2t 3=0 ⇒ t =3/2 When t =3/2,y =13/2 and z =7/2 y =3t +2=0 ⇒ t = 2/3, so x = 13/3 and z =17/3 z =5 t =0 ⇒ t =5, so x =7 and y =17.

Thepointsare(0,13/2,7/2), ( 13/3, 0, 17/3),and(7,17,0).

36. Wecouldshowthattwopointsonthelinearealsointheplaneorthatforpointsontheline: 2x y +4z =2(5 t) (2t 7)+4(t 3)=5,sotheyareintheplane.

37. Forpointsonthelineweseethat x 3y + z =(5 t) 3(2t 3)+(7t +1)=15,sothelinedoesnotintersecttheplane.

38. Firstweparametrizethelinebysetting t =(x 3)/6,whichgivesus x =6t +3, y =3t 2, z =5t.Pluggingthese parametricvaluesintotheequationfortheplanegives 2(6t +3) 5(3t 2)+3(5t)+8=0 ⇐⇒ 12t +24=0 ⇐⇒ t = 2.

Theparametervalue t = 2 yieldsthepoint (6( 2)+3, 3( 2) 2, 5( 2))=( 9, 8, 10)

39. We findparametricequationsforthelinebysetting t =(x 3)/( 2),sothat x =3 2t, y = t +5, z =3t 2.Plugging theseparametricvaluesintotheequationfortheplane,we findthat 3(3 2t)+3(t +5)+(3t 2)=9 6t +3t +15+3t 2=22 for all valuesof t.Hencethelineiscontainedintheplane.

40. Againwe findparametricequationsfortheline.Set t =(x +4)/3,sothat x =3t 4, y =2 t, z =1 9t.Plugging theseparametricvaluesintotheequationfortheplane,we findthat

2(3t 4) 3(2 t)+(1 9t)=7 ⇐⇒ 6t 8 6+3t +1 9t =7 ⇐⇒−13=7

Hencewehaveacontradiction;thatis, no valueof t willyieldapointonthelinethatisalsoontheplane.Thusthelineand theplanedo not intersect.

41. Wejustplugtheparametricexpressionsfor x, y , z intotheequationforthesurface:

forallvaluesof t ∈ R.Henceallpointsonthelinesatisfytheequationforthesurface.

42. Asexplainedinthetext,wecan’tjustsetthetwosetsofequationsequaltoeachotherandsolve.Ifthetwolinesintersectata point,wemaygettothatpointattwodifferenttimes.Let’scallthesetimes t1 and t2 andsolvetheequations

Eliminate t1 bysubtractingthethirdequationfromthe firsttoget t2 =2.Substitutebackintoanyoftheequationstoget t1 = 1.Usingeithersetofequations,you’ll findthatthepointofintersectionis (1, 0, 1)

43. Thewaytheproblemisphrasedtipsusoffthatsomethingisgoingon.Let’shandlethisthesamewaywedidinExercise42.

Addingthelasttwoequationseliminates t2 andgivesus t1 =13/2.Thiscorrespondstothepoint(14, 39/2,11/2). Substitutingthisvalueof t1 intothethirdequationgivesus t2 =3/2,whilesubstitutingthisintothe firstequationgivesus t2 =13/3.Thisinconsistencytellsusthatthesecondlinedoesn’tpassthroughthepoint (14, 39/2, 11/2).

44.(a) Thedistanceis (3t 5+2)2

(b) Usingastandard firstyearcalculustrick,thedistanceisminimizedwhenthesquareofthedistanceisminimized.Sowe find D =26t2 2t +13 isminimized(atthevertexoftheparabola)when t =1/26.Substitutebackintoouranswer for(a)to findthattheminimaldistanceis 337/26

45.(a) AsinExample2,thisistheequationofacircleofradius2centeredattheorigin.Thedifferenceisthatyouaretraveling arounditthreetimesasfast.Thismeansthatif t variedbetween0and 2π thatthecirclewouldbetracedthreetimes.

(b) Thisisjustlikepart(a)excepttheradiusofthecircleis5.

(c) Thisisjustlikepart(b)exceptthe x and y coordinateshavebeenswitched.Thisisthesameasreflectingthecircleabout theline y = x andsothisisalsoacircleofradius5.Ifyoucare,thecirclein(b)wasdrawnstartingatthepoint(5,0) counterclockwisewhilethiscircleisdrawnstartingat(0,5)clockwise.

(d) Thisisanellipsewithmajoraxisalongthe x-axisintersectingitat (±5, 0) andminoraxisalongthe y -axisintersectingit at (0, ±3): x

46. Thediscussioninthetextofthecycloidlookedatthepathtracedbyapointonthecircumferenceofacircleofradius a asitis rolledwithoutslippingonthe x-axis.Thevectorfromtheorigintoourpoint P wassplitintotwopieces: → OA (thevectorfrom theorigintothecenterofthecircle)and → AP (thevectorfromthecenterofthecircleto P ).Thissplitremainsthesameinour problem.

Chapter1Vectors

Thecenterofthecircleisalways a abovethe x-axis,andafterthewheelhasrolledthroughacentralangleof t radiansthe x coordinateisjust at.So → OA =(at,a).Thisdoesnotchangeinourproblem.

Thevector → AP wascalculatedtobe ( a sin t, a cos t).Thedirectionofthevectorisstillcorrectbutthelengthisnot.If weare b unitsfromthecenterthen → AP = b(sin t, cos t)

Weconcludethenthattheparametricequationsare x = at b sin t,y = a b cos t.When a = b thisisthecaseofthe cycloiddescribedinthetext;when a>b wehavethecurtatecycloid;andwhen a<b wehavetheprolatecycloid.

Forapictureofhowtogenerateoneconsiderthediagram:

Heretheinnercircleisrollingalongthegroundandtheprolatecycloidisthepathtracedbyapointontheoutercircle. Thereisaclassictoywithaplasticwheelthatrunsalongahandheldtrack,butyourstudentsaretooyoungforthat.Youcould pretendthatthebigcircleistheendofaroundroastandthelittlecircleistheendofaskewer.Inaregularrotisserietheroast wouldjustrotateontheskewer,butwecouldimaginerollingtheskeweralongtheedgesofthegrill.Themotionofapointon theoutsideoftheroastwouldbeaprolatecycloid.

47. YouaretopicturethatthecirculardispenserstaysstillsoEgberthastounwindaroundthedispenser.Thedirectionis (cos θ, sin θ ).Thelengthistheradiusofthecircle a,plustheamountoftapethat’sbeenunwound.Thetapethat’sbeen unwoundisthedistancearoundthecircumferenceofthecircle.Thisis aθ where θ isagaininradians.Theequationis therefore (x,y )= a(1+ θ )(cos θ, sin θ )

1.3TheDotProduct

Exercises1–16arejuststraightforwardcalculations.For1–6useDefinition3.1andformula(1).For7–11useformula(4).For 12–16useformula(5)

1. (1, 5) ( 2, 3)=1( 2)+5(3)=13, (1, 5) = √12 +52 = √26, ( 2, 3) = ( 2)2 +32 = √13

2. (4, 1) · (1/2, 2)=4(1/2) 1(2)=0, (4, 1) = 42 +( 1)2 = √17 (1/2, 2) = (1/2)2 +22 = √17/2

3. ( 1, 0, 7) (2, 4, 6)= 1(2)+0(4)+7( 6)= 44, ( 1, 0, 7) = ( 1)2 +02 +72 = √50=5√2,and (2, 4, 6) = 22 +42 +( 6)2 = √56=2√14

4. (2, 1, 0) (1, 2, 3)=2(1)+1( 2)+0(3)=0, (2, 1, 0) = √22 +1= √5,and (1, 2, 3) = 12 +( 2)2 +32 = √14.

5. (4i 3j + k) (i + j + k)=4(1)+ 3(1)+1(1)=2, 4i 3j + k = √42 +32 +12 = √26,and i + j + k = √1+1+1= √3.

6. (i +2j k) ( 3j +2k)=2( 3) 1(2)= 8, i +2j k = 12 +22 +( 1)2 = √6,and − 3j +2k = ( 3)2 +22 = √13

7. θ =cos 1 (√3i + j) ( √3i + j) (√3i + j) − √3i + j =cos 1 3+1 (2)(2) =cos 1 1 2 = 2π 3 .

8. θ =cos 1 ( 1, 2) (3, 1) ( 1, 2) (3, 1) =cos 1 3+2 √5 √10 =cos 1 1 5√2

9. θ =cos 1 (i + j) · (i + j + k) i + j i + j + k =cos

10. θ =cos 1 (i + j k) · ( i +2j +2k) i + j k − i +2j +2k =cos

11. θ =cos 1 (1, 2, 3) · (3, 6, 5) (1, 2, 3) (3, 6, 5) =cos 1 3+12 15 √

Note:Theanswersto12and13arethesame.Youmaywanttoassignbothexercisesandaskyourstudentswhythisshouldbe true.Youmightthenwanttoaskwhatwouldhappenifvector a wasthesamebutvector b wasdividedby √2

12. proji+j (2i +3j k)= (i + j) (2i +3j k) (i + j) (i + j) (i + j)= 2+3 1+1 (1, 1, 0)= 5 2 , 5 2 , 0

13. proj i+j √2 (2i +3j k)= ⎛

i+j √2 (2i +3j k) i+j √2 · i+j √2

14. proj5k (i j +2k)= (5k) · (i j +2k) (5k) · (5k) (5k)=

15. proj 3k (i j +2k)= ( 3k) · (i j +2k) ( 3k) · ( 3k) ( 3k)= 6 9 ( 3k)=2k

, 1, 0) √2 = 5 2 , 5 2 , 0

16. proji+j+2k (2i 4j + k)= (i + j +2k) (2i 4j + k) (i + j +2k) (i + j +2k) (i + j +2k)= 2 4+2 1+1+4 (1, 1, 2)=0.

17. Wejustdividethevectorbyitslength: 2i j + k ||2i j + k|| = 1 √6 (2, 1, 1)

18. Herewetakethenegativeofthevectordividedbyitslength: i 2k i 2k = 1 √5 (1, 0, 2).

19. SameideaasExercise17,butmultiplyby3: 3(i + j k) i + j k = 3 √3 (1, 1, 1)= √3(1, 1, 1).

20. Thereareawholeplanefullofperpendicularvectors.Theeasiestthreeto findarewhenwesetthecoefficientsofthecoordinate vectorsequaltozerointurn: i + j, j + k,and i + k

21. Wehavetwocasestoconsider.

Ifeitheroftheprojectionsiszero:proja b = 0 ⇔ a

projb a = 0 Ifneitheroftheprojectionsiszero,thenthedirectionsmustbethesame.Thismeansthat a mustbeamultipleof b.Let a = cb,thenontheonehand

Ontheotherhand

Theseareequalonlywhen c =1

Inotherwords,proja b = projb a

22. Property2:

23. Wehave

24. Thefollowingdiagramsmightbehelpful: a

To find F1 ,thecomponentof F inthedirectionof a,weproject F onto a:

1 = proja F = (i 2j) (4i + j) (4i + j) (4i + j) (4i + j)= 2 17 (4, 1)

To find F2 ,thecomponentof F inthedirectionperpendicularto a,wecanjustsubtract F1 from F: F2 =(1, 2) 2 17 (4, 1)= 9 17 , 36 17 = 9 17 (1, 4)

Notethat F1 isamultipleof a sothat F1 doespointinthedirectionof a andthat F2 a =0 so F2 isperpendicularto a.

25.(a) Theworkdonebytheforceisgiventobetheproductofthelengthofthedisplacement( → PQ )andthecomponentof forceinthedirectionofthedisplacement(± proj → PQ F orinthecasepicturedinthetext, F cos θ ).Thatis,

Work = → PQ F cos θ = F · → PQ

usingTheorem3.3. (b) Thedisplacementvectoris → PQ = i + j 2k andso,usingpart(a),wehave

26. Theamountofworkis

27. Tomovethebananas,onemustexertan upward forceof500lb.Suchaforcemakesanangleof 60◦ withtheramp,anditis therampthatgivesthedirectionofdisplacement.Thustheamountofworkdoneis

28. Notethat i, j,and k eachpointalongthepositive x-, y -,and z -axes.Therefore,wemayuseTheorem3.3tocalculatethat

29. Asinthepreviousproblem,weuse a =3i +4k to findthat

α = (3i +4k) · i 3i +4k (1) = 3 5 ;

β = (3i +4k) · j 3i +4k (1) =0;

γ = (3i +4k) · k 3i +4k (1) = 4 5

30. Youcouldeitherusethethreerighttrianglesdeterminedbythevector a andthethreecoordinateaxes,oryoucoulduse Theorem3.3.Bythattheorem, cos α = a i a i = a1 a2 1 + a2 2 + a2 3 .Similarly, cos β = a2 a2 1 + a2 2 + a2 3 and cos γ = a3 a2 1 + a2 2 + a2 3

31. Considerthe figure:

If P1 isthepointon AB located r timesthedistancefrom A to B ,thenthevector → AP1 = r → AB.Similarly,since P2 isthepoint on AC located r timesthedistancefrom A to C ,thenthevector → AP2 = r → AC.Sonowwecanlookatthelinesegment P1 P2 usingvectors.

Thetwoconclusionsnowfollow.Because → P1 P2 isascalarmultipleof → BC,theyareparallel.Alsothepositivescalar r pulls outofthenormso → P1 P2 = r → BC = r → BC .

32. ThisnowfollowsimmediatelyfromExercise31orExample6fromthetext.Consider firstthetriangle ABC.

If M1 isthemidpointof AB and M2 isthemidpointof BC ,we’vejustshownthat M1 M2 isparallelto AC andhas halfitslength.Similarly,considertriangle DAC where M3 isthemidpointof CD and M4 isthemidpointof DA.Wesee that M3 M4 isparallelto AC andhashalfitslength.The firstconclusionisthat M1 M2 and M3 M4 havethesamelengthand areparallel.Repeatthisprocessfortriangles ABD and CBD toconcludethat M1 M4 and M2 M3 havethesamelengthand areparallel.Weconcludethat M1 M2 M3 M4 isaparallelogram. Forkicks—haveyourstudentsdrawthe figureforABCDa non-convexquadrilateral.Theargumentandtheconclusionstillholdeventhoughoneofthe“diagonals”isnotinsideofthe quadrilateral.

33. Inthediagraminthetext,thediagonalrunningfromthebottomlefttothetoprightis a + b andthediagonalrunningfrom thebottomrighttothetopleftis b a.

Sinceneither a nor b iszero,theymustbeorthogonal.

34. UsingthesamesetupasthatinExercise33,wenote firstthat

Itfollowsimmediatelythat

Inotherwordsthatthediagonalsoftheparallelogramareperpendicularifandonlyiftheparallelogramisarhombus.

35.(a) Let’sstartwiththetwocircleswithcentersat W1 and W2 .Assumethatinadditiontotheirintersectionatpoint O that theyalsointersectatpoint C asshownbelow.

Thepolygon OW1 CW2 isaparallelogram.Infact,becauseallsidesareequal,itisarhombus.Wecan,therefore,write thevector

.Similarly,wecanwrite

(b) Let’susetheresultsofpart(a)togetherwiththehint.Weneedtoshowthatthedistancefromeachofthepoints A, B , and C to P is r .Let’sshow,forexample,that → CP is r :

Theargumentsfortheothertwopointsareanalogous.

(c) Whatwereallyneedtoshowisthateachofthelinespassingthrough O andoneofthepoints A, B ,or C isperpendicularto thelinecontainingthetwootherpoints.Usingvectorswewillshowthat

byshowing theirdotproductsare0.It’senoughtoshowthisforoneofthem:

36.(a) ThisfollowsimmediatelyfromExercise34ifyounoticethatthevectorsarethediagonalsoftherhombuswithtwosides b a and a b

Orwecanproceedwiththecalculation: ( b a + a ) · ( b a − a b).Theonlybitofgoodnewshereisthatthe crosstermsclearlycanceleachotheroutandwe’releftwith: b

=0 (b) Asin(a),theslickerwayistorecall(orreprovegeometrically)thatthediagonalsofarhombusbisectthevertexangles. Thennotethat ( b a + a b) isthediagonaloftherhombuswithsides b a and a b andsobisectstheanglebetween themwhichisthesameastheanglebetween a and b

Anotherwayistolet θ1 betheanglebetween a and b a + a b,andlet θ2 betheanglebetween b and b a + a b Then

Also

So b a + a b bisectstheanglebetweenthevectors a and b

1.4TheCrossProduct

ForExercises1–4useDefinition4.2

1. (2)(3) (4)(1)=2

2. (0)(6) (5)( 1)=5

3. (1)(2)(3)+(3)(7)( 1)+(5)(0)(0) (5)(2)( 1) (1)(7)(0) (3)(0)(3)= 5

4. ( 2)(6)(2)+(0)( 1)(4)+(1/2)(3)( 8) (1/2)(6)(4) ( 2)( 1)( 8) (0)(3)(2)= 32

Note:InExercises5–7,thedifferencebetweenusing(2)and(3)reallyamountstochangingthecoefficientof j from (a3 b1 a1 b3 ) informula(2)to (a1 b3 a3 b1 ) informula(3).ThedetailsareonlyprovidedinExercise5

5. Firstwe’lluseformula(2): (1, 3, 2) × ( 1, 5, 7)=[(3)(7) ( 2)(5)]i +[( 2)( 1) (1)(7)]j +[(1)(5) (3)( 1)]k =31i 5j +8k =(31, 5, 8) Ifinsteadweuseformula(3),weget: (1, 3, 2) × ( 1, 5, 7)= ijk 13 2 157 = 3 2 57 i 1 2 17 j + 13 15 k =31i 5j +8k =(31, 5, 8)

6. Justusingformula(3): (3i 2j + k) × (i + j + k)= ijk 3 21

= 21 11 i 31 11 j + 3 2 11 k = 3i 2j +5k =( 3, 2, 5).

7. Notethatthesetwovectorsformabasisforthe xy -planesothecrossproductwillbeavectorparallelto(0,0,1).Again,just usingformula(3): (i + j) × ( 3i +2j)= ijk 110

, 0, 5)

8. By(1) (a + b) × c = c × (a + b)

By(2),this = c × a + c × b

By(1),this = a × c + b × c.

9. (a + b) × (a b)=(a × a)+(b × a) (a × b) (b × b).Thecrossproductofavectorwithitselfis 0 andalso (b × a)= (a × b),so (a + b) × (a b)= 2(a × b).

Youmaywishtohaveyourstudentsconsiderwhatthismeansabouttherelationshipbetweenthecrossproductofthesidesofa parallelogramandthecrossproductofitsdiagonals.Inanycase,wearegiventhat a×b =(3, 7, 2),so (a+b)×(a b)= ( 6, 14, 4).

10. Ifyouplotthepointsyou’llseethattheyaregiveninacounterclockwiseorderoftheverticesofaparallelogram.To findthe areawewillviewthesidesfrom(1,1)to(3,2)andfrom(1,1)to ( 1, 2) asvectorsbycalculatingthedisplacementvectors: (3, 2) (1, 1) and ( 1, 2) (1, 1).Wethenembedtheproblemin R3 andtakeacrossproduct.Thelengthofthiscross productistheareaoftheparallelogram.

11. Thisistricky,asthepointsarenotgiveninorder.The figureontheleftshowsthesidesconnectedintheorderthatthepoints aregiven.

Asthe figureontherightshows,ifyoutakethe firstsidetobethesidethatjoinsthepoints(1,2,3)and (4, 2, 1) then thenextsideisthesidethatjoins (4, 2, 1) and (0, 3, 2).Wewillagaincalculatethelengthofthecrossproductofthe displacementvectors.Sotheareaoftheparallelogramwillbethelengthof

12. Thecrossproductwillgiveustherightdirection;ifwethendividethisresultbyitslengthwewillgetaunitvector: (2, 1, 3) × (1, 0, 1) (2, 1, 3) × (1, 0, 1) = (1, 5, 1) (1, 5, 1) = 1 √27 (1, 5, 1)

13. For (a × b) c tobezeroeither

• Oneormoreofthethreevectorsis 0,

• (a × b)= 0 whichwouldhappenif a = k b forsomereal k ,or

• c isintheplanedeterminedby a and b

ForExercises14–17we’lljusttakehalfofthelengthofthecrossproduct.UnlikeExercises10and11,inExercises16and17 wedon’thavetoworryabouttheorderingofthepoints.Inatriangle,whicheverorderwechoosewearetravelingeitherclockwise orcounterclockwise.Justchooseanyoftheverticesasthebaseforthecrossproduct.Ourchoicesmaydiffer,butthesolution won’t

14. (1/2) (1, 1, 0) × (2, 1, 0) =(1/2) (0, 0, 3) =3/2.

15. (1/2) (1, 2, 6) × (4, 3, 1) =(1/2) ( 16, 25, 11) = √1002/2.

16. (1/2) ( 1 1, 2 1, 0) × ( 2 1, 1 1, 0) =(1/2) ( 2, 1, 0) × ( 3, 2, 0) = (1/2) (0, 0, 7) =7/2.

17. (1/2) (0 1, 2, 3 1) × ( 1 1, 5, 2 1) =(1/2) ( 1, 2, 2) × ( 2, 5, 3) = (1/2) ( 16, 7, 1) = √306/2=3√34/2.

ThetriplescalarproductisusedinExercises18and19andtheequivalentdeterminantformmentionedinthetextisproved inExercise20

Somepeoplewritethisproductas a (b × c) insteadof (a × b) c.Exercise28showsthattheseareequivalent.

18. Herewearegiventhevectorssowecanjustusethetriplescalarproduct:

Volume = |(a × b) · c| =3

19. Youneedto figureoutausefulorderingofthevertices.Youcaneitherplotthembyhandoruseacomputerpackagetohelp oryoucanmakesomeobservationsaboutthem.Firstlookatthe z coordinates.Twopointshave z = 1 andtwohave z =0 Theseformyourbottomface.Oftheremainingpointstwohave z =5—thesewillmatchupwiththebottompointswith z = 1,andtwohave z =6—thesewillmatchupwiththebottompointswith z =0.Theparallelepipedisshownbelow. We’llusethehighlightededgesasourthreevectors a, b,and c.Youcouldhavebasedthecalculationatanyvertex.Ihave chosen (4, 2, 1).Thethreevectorsare:

Finally,Volume = |(a × b) c| =53

Note:TheproofsofExercises20and28areeasierifyourememberthatifmatrix A isjustmatrix B withanytworows interchangedthenthedeterminantof A isthenegativeofthedeterminantof B .Ifyoudon’tusethisfact(whichisexploredin exerciseslaterinthischapter),youcanprovethiswithalongcomputation.Thatiswhytheauthorofthetextsuggeststhata computeralgebrasystemcouldbehelpful—andthiswouldbeagreatplacetouseitinaclassdemonstration.

20. Thisisnotasbadasitmight firstappear.

×

) · c = ⎛ ⎝ ijk

21. (a × b) c =

byExercise20.Similarly,

byExercise20. Expandthesedeterminantstoseethattheyareequal.

22. Thevalueof |(a × b) c| isthevolumeoftheparallelepipeddeterminedbythevectors a, b, c.Butsois |b (a × c)|,sothe quantitiesmustbeequal.

23.(a) Wehave

Expandingandtakingabsolutevalue,weobtain

Fromhere,itseasytoseethatthisagreeswiththeformulaabove.

(b) Wecomputetheabsolutevalueof

Thustheareais 1 2 = 1 2

24. Surfacearea

25. Weassumethat a, b,and c arenon-zerovectorsin R3

(a) Thecrossproduct a × b isorthogonaltoboth a and b

(b) Scalethecrossproducttoaunitvectorbydividingbythelengthandthenmultiplyby2toget 2 a × b a × b

(c) proja b = a · b a · a a

(d) Herewedividevector a byitslengthandmultiplyitbythelengthof b toget b a a

(e) Thecrossproductoftwovectorsisorthogonaltoeach: a × (b × c)

(f) Avectorperpendicularto a × b willbebackintheplanedeterminedby a and b,soouransweris (a × b) × c

26. I lovethisproblem—studentstendtogoaheadandcalculatewithoutthinkingthroughwhatthey’redoing first.Thiswould makeagreatquizatthebeginningofclass

(a) Vector:Thecrossproductofthevectors a and b isavectorsoyoucantakeitscrossproductwithvector c

(b) Nonsense:Thedotproductofthevectors a and b isascalarsoyoucan’tdotitwithavector.

(c) Nonsense:Thedotproductsresultinscalarsandyoucan’t findthecrossproductoftwoscalars.

(d) Scalar:Thecrossproductofthevectors a and b isavectorsoyoucantakeitsdotproductwithvector c

(e) Nonsense:Thecrossproductofthevectors a and b isavectorsoyoucantakeitscrossproductwithvectorthatisthe resultofthecrossproductof c and d

(f) Vector:Thedotproductresultsinascalarthatisthenmultipliedbyvector d.Wecanevaluatethecrossproductofvector a withthisresult.

(g) Scalar:Wearetakingthedotproductoftwovectors.

(h) Vector:Youaresubtractingtwovectors.

Note:Youcanhaveyourstudentsuseacomputeralgebrasystemfortheseassuggestedinthetext.I’veincludedworkedout solutionsforthoseasoldfashionedasIam

27. Exercise25(f)showsusthat (a × b) × c isintheplanedeterminedby a and b andsoweexpectthesolutiontobeoftheform k1 a + k2 b forscalars k1 and k2 Usingformula(3)fromthetextfor a × b:

Look firstatthecoefficientof

andregroupwehave:

.Expandthen addandsubtract

,expandthenaddandsubtract a3 b3 c3 andregrouptoobtain

Nowhere’saversionofExercise27workedon Mathematica.Firstyouenterthefollowingtodefinethevectors a, b,and c

a = {a1,a2,a3}

b = {b1,b2,b3}

c = {c1,c2,c3}

Thereplyfrom Mathematica isanechoofyourinputfor c.Let’sbeginbycalculatingthecrossproduct.Youcaneitherselect thecrossproductoperatorfromthetypesettingpaletteoryoucantypetheescapekeyfollowedby“cross”followedbythe escapekey. Mathematica shouldreformatthiskeysequenceas × andyoushouldbeabletoenter

(a × b) × c.

Mathematica willrespondwiththecalculatedcrossproduct

Nowyoucanchecktheotherexpression.Useaperiodforthedotinthedotproduct.

Mathematica willimmediatelyrespond

Thiscertainlylooksdifferentfromthepreviousexpression.Beforegivinguphope,notethatthisonehasbeenfactoredand theearlieronehasnot.Youcanexpandthisbyusingthecommand Expand[(a.c)b (

oruse Mathematica’scommand%torefertothepreviousentryandjusttype

Expand[%]

Thisstillmightnotlookfamiliar.Sotakealookat

Ifthis still isn’twhatyouarelookingfor,simplifyitwiththecommand

Simplify[%]

and Mathematica willrespond {0, 0, 0}.

28. Theexerciseasksustoshowthatsixquantitiesareequal.

Themostimportantpairis a (b × c)= c (a × b).Becauseofthecommutativepropertyofthedotproduct c (a × b)= (a × b) c andsoweareshowingthat a (b × c)=(a × b) c c (a × b)=(a × b) c

Thedeterminantsofthe3by3matricesaboveareequalbecausewehadtointerchangetworowstwicetogetfromoneto theother.Thisfacthasnotyetbeenpresentedinthetext.Thiswouldbeanexcellenttimetouseacomputeralgebrasystem toshowthetwodeterminantsareequal.Ofcourse,youcoulduse Mathematica orsomeothersuchsystemtodotheentire problem.

Toshowthat a (b × c)= b (c × a) weuseasimilarapproach:

Sowe’veestablishedthatthe firstthreetriplescalarsareequal. Wegettherestalmostforfreebynoticingthatthreepairsofequationsaretrivial:

Eachoftheabovepairsareequalbytheanticommutativitypropertyofthecrossproduct.Ifyoupreferthematrixapproach, thisalsofollowsfromthefactthatinterchangingtworowschangesthesignofthedeterminant.

29. ByExercise28, (a × b) · (c × d)= c · (d × (a × b))

Byanticommutativity, c (d × (a × b))= c ((a × b) × d)

ByExercise27, c ((a × b) × d)= c ((a d)b (b d)a)=(

30. ApplytheresultsofExercise27toeachofthethreecomponents:

+[(c b)a (a b)c]= 0.

(Forexample,the (a c)b cancelswiththe (c a)b becauseofthecommutativepropertyforthedotproduct.)

31. Ifyourstudentsareusingacomputeralgebrasystem,theymaynotnoticethatthisis exactly thesameproblemasExercise27. Justreplace c with (c × d) onbothsidesoftheequationinExercise27toobtaintheresulthere.

32. FirstapplyExercise29tothedotproducttoget

Youcaneitherobservethattwoofthesequantitiesmustbe0,oryoucanapplyExercise28tosee a · (c × a)= c · (a × a)=0 Exercise28alsoshowsthat b (c × a)= a (b × c).Theresultfollows.

33. WedidthisaboveinExercise29.

34. Theamountoftorqueistheproductofthelengthofthe“wrench”andthecomponentoftheforceperpendiculartothe “wrench”.Inthiscase,thewrenchisthedoor—sothelengthisfourfeet.The20lbforceisappliedperpendiculartothe planeofthedoorwayandthedoorisopen 45◦ .Sofromthetext,theamountoftorque = a F sin θ =(4)(20)(√2/2)= 40√2 ft-lb.

35.(a) Herethelengthof a is1foot,theforce F = 40poundsandangle θ = 120degrees.So

Torque =(1)(40)sin120◦ =40 √3 2 =20√3 foot-pounds

(b) Hereallthathaschangedisthat a is1.5feet,so

Torque =(3/2)(40)sin120◦ =60 √3 2 =30√3 foot-pounds

36. a =2 inbuttorqueismeasuredinfoot-poundsso a =(1/6) ft.

Torque = a × F = 1 6 , 0, 0 × (0, 15, 0)= 0, 0, 5 2

SoEgbertisusing5/2foot-poundsstraightup.

37. Fromthe figure

Thisistheangletheseesawmakeswithhorizontal.Theanglewewantis

Since r =3 and F =50,theamountoftorqueis T = r × F = r F sin 2π 3 =3 50 √3 2 =75√3 ft-lb 1.5 3 F r 2 /3 F (translated)

38.(a) Thelinearvelocityis v = ω × r sothat v = ω r sin θ.

Wehavethattheangularspeedis 2π radians 24hrs = π 12 radians/hr (thisis ω .)Also r =3960,soat 45◦ Northlatitude, v = π 12 · 3960 · sin45◦ = 330π √2 ≈ 733.08mph.

(b) Heretheonlychangeisthat θ =90◦ .Thus v = π 2 3960 sin90◦ =330π ≈ 1036.73mph.

39. Archie’sactualexperienceisn’timportantinsolvingthisproblem;hecouldhaveriddenclosertothecenter.Sinceweareonly interestedincomparingArchie’sexperiencewithAnnie’s,itturnsoutthattheirdifferencewouldbethesamesolongasthedifferenceintheirdistancefromthecenterremainedat2inches.Thedifferenceinspeedis (331/3)(2π )(6) (331/3)(2π )(4)= (331/3)(2π )(2)=4π (331/3)=1331/3π =400π/3in/ min

40.(a) v = ω × r =(0, 0, 12) × (2, 1, 3)=(12, 24, 0)=12i +24j

(b) Theheightofthepointdoesn’tchangesowecanviewthisasifitwereaproblemin R2 .When x =2 and y = 1,we can findthecentralanglebytaking tan 1 ( 1/2).Inonesecondtheanglehasmoved12radianssothenewpointis (√5cos(tan 1 ( 1/2)+12), √5sin(tan 1 ( 1/2)+12), 3) ≈ (1 15, 1 92, 3)

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