PROGRESSIONS
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GEOMETRY Arithmetical Progression An arithmetical progression is a series of numbers in which each consecutive term differs from the preceding one by a fixed amount called the common difference, d. Thus, 1, 3, 5, 7, etc., is an arithmetical progression where the difference d is 2. The difference here is added to the preceding term, and the progression is called increasing. In the series 13, 10, 7, 4, etc., the difference is ( −3), and the progression is called decreasing. In any arithmetical progression (or part of progression), let a =first term considered l =last term considered n =number of terms d =common difference S =sum of n terms Then the general formulas are l = a + ( n – 1 )d
and
a+l S = ----------- × n 2
In these formulas, d is positive in an increasing and negative in a decreasing progression. When any three of the preceding live quantities are given, the other two can be found by the formulas in the accompanying table of arithmetical progression. Example:In an arithmetical progression, the first term equals 5, and the last term 40. The difference is 7. Find the sum of the progression. a+l 5 + 40 S = ----------- ( l + d – a ) = --------------- ( 40 + 7 – 5 ) = 135 2d 2×7 Geometrical Progression A geometrical progression or a geometrical series is a series in which each term is derived by multiplying the preceding term by a constant multiplier called the ratio. When the ratio is greater than 1, the progression is increasing; when less than 1, it is decreasing. Thus, 2, 6, 18, 54, etc., is an increasing geometrical progression with a ratio of 3, and 24, 12, 6, etc., is a decreasing progression with a ratio of 1⁄2. In any geometrical progression (or part of progression), let a =first term l =last (or nth) term n =number of terms r =ratio of the progression S =sum of n terms Then the general formulas are l = ar n – 1
and
rl – a S = ------------r–1
When any three of the preceding five quantities are given, the other two can be found by the formulas in the accompanying table. For instance, geometrical progressions are used for finding the successive speeds in machine tool drives, and in interest calculations. Example:The lowest speed of a lathe is 20 rpm. The highest speed is 225 rpm. There are 18 speeds. Find the ratio between successive speeds. n–1
Ratio r =
l --- = a
17
225 --------- = 20
17
11.25 = 1.153