Product of sum by difference application

We know the formula

$$x^2-y^2=(x-y)(x+y)$$

This formula gives us opportunity to find some product .
First we prepare ourselves to apply former formula in the opposite direction.

$$(x-y)(x+y)=x^2-y^2$$

$ 19*21=(20-1)(20+1)=20^2-1=400-1=399 $
$ 29*31=(30-1)(30+1)=30^2-1=900-1=899 $
$ 39*41=(40-1)(40+1)=40^2-1=1600-1=1599 $
$ 49*51=(50-1)(50+1)=50^2-1=2500-1=2499 $

Average: 4 (5 votes)

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