## Sunday, July 29, 2012

### Multiplying Complementary Pairs

Multiplying Complementary Pairs
Quick! What's 23 x 27?
621
There's a trick to doing this quickly. Can you see a pattern in these
multiplications?
42 x 48 = 2016
43 x 47 = 2021
44 x 46 = 2024
54 x 56 = 3024
64 x 66 = 4224
61 x 69 = 4209
111 x 119 = 13209
In each pair above, the numbers being multiplied are *complementary*: they
are the same number except for the rightmost digit, and the rightmost
digits add to 10.
The trick to multiplying complementary pairs is to take the rightmost
digits and multiply them; the result forms the two rightmost digits of the
answer. (So in the last example 1 x 9 = 09.) Then take the first number
without its rightmost digit, and multiply it by the next higher whole
number; the result forms the initial digits of the answer. (So in the last
example: 11 x 12 = 132. Voila! The answer is 13209.)
*The Math Behind the Fact:*
This trick works because you are multiplying pairs of numbers of the form
10*N+A and 10*(N+1)-A, where N is a whole number and A is a digit between 1
and 9. A little algebra shows their product is:
100*N*(N+1) + A*(10-A).
The first term in the sum is a multiple of 100 and it does not interact
with the last two digits of sum, which is never more than two digits long.

Subscribe to:
Post Comments (Atom)

## No comments:

## Post a Comment