Multiply Two Numbers Having Difference of Two

This is a quick way to multiply the numbers that differ by 2. It works best if the first number ends in 9.For example 19x21, 29x31, 39x41, 69x71…etc.

How to do it?

1. Find the middle integer.
2. Square of the middle integer.
3. Subtract 1 from the step2.
For Example,

Multiply 29 by 31.
Middle number is 30.
Square of 30 =900.
900 -1 = 899.

Another example
15 x 17.
Middle number is 16.
Answer to 15 x 17 = 162-1 = 255.