SOLUTION: A friend claims that by writing the product 19x21 as (20-1)(20 + 1), the product can be computed mentally. How is this possible?

Question 626854: A friend claims that by writing the product 19x21 as
(20-1)(20 + 1), the product can be computed mentally. How is this possible?
Found 2 solutions by jim_thompson5910, reviewermath:
Answer by jim_thompson5910(35256) (Show Source): You can put this solution on YOUR website!
(20-1)(20 + 1) = 20^2 - 1^2 = 400 - 1 = 399

So 19 x 21 = 399

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Answer by reviewermath(1029) (Show Source): You can put this solution on YOUR website!
The product of the sum and difference is equal to the difference between the squares.
In symbol: .
Let a = 20 and b = 1 so
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