Chapter 12 Discovering Systematic Procedures
4 Move the rod representing the second number to the right so that the space for the difference is seen on the left. 5 Now try to read the difference without using rods to fill the space. The difference is easily seen to be 31 in the first example and fairly easily to be 28 in the second. It may be necessary for some pupils to use a rod for the units figure in the second instance (another tan rod in this case); use can be made of this difficulty to introduce the third step, which involves subtractions where the addition of a value to both numbers allows the difference to be easily read. 1
Make the pattern for 56 â€“ 38 again.
2 What must be added to 38 to make 40? 3 Add 2 to both numbers. 4 Can you read the difference quite easily now? This series of activities should be attempted by second grade pupils. Of course, finding the difference by means of the rods should precede exercises for which rods are not used. The exercises involving the larger numbers should later be carried out with crosses for the patterns instead of rods end to end. Finally, differences should be found without using any rods at all. When this stage is reached, the operation will have been thoroughly understood.
Published on Nov 18, 2009