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With this method you only ever need multiplication tables up to 5 times 5. It is one of many ancient Indian sutras and this one involves a cross subtraction method which, according to old historical traditions, is responsible for the acceptance of the ´ mark as the sign of multiplication. Here is a very simple example of the method. Can you give a good explanation of WHY it works?
Suppose we want to multiply 9 by 7. We subtract each number from 10 and, using these differences (or deficiencies), write:
9-1 |
7-3 |
6 3 |
The product has two parts, left and right.
To get the right part (or units digit) multiply the deficiencies
(1×3)
The left hand digit (tens digit) of the answer can be found in four
different ways. Why do they all give the same answer?
This gives the answer 63.
Here are some more examples. Try some of your own.
9-1 | 8-2 | 9-1 | 8-2 |
6-4 | 7-3 | 9-1 | 5-5 |
5 4 | 5 6 | 8 1 | 4 0 |
Note: Here you have to express 5 times 2 as 1 ten and 0 units. |