# Product of two one-digit numbers greater than 5

This is a rule that helps remember a big part of the multiplication table. Assume you forgot the product 7·9. Do this. First find the excess of each of the multiples over 5: it's 2 for 7

The explanation comes from the following formula:

(5 + a)(5 + b) = 10(a + b) + (5 - a)(5 - b)

In our example, a = 2 and b = 4.

Another example:

8×9 | = (5 + 3)(5 + 4) | |

= 10(3 + 4) + (5 - 3)(5 - 4) | ||

= 70 + 2×1 | ||

= 72. |

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