Evaluate the following using the suitable identity; 21*19
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21 can be written as (20+1)
and 19 can be written as (20-1)
(20+1) × (20-1)
Using the identity (a+b) × (a-b) = a^2 - b^2
(20)^2 - (1)^2 = 400 - 1 = 399
and 19 can be written as (20-1)
(20+1) × (20-1)
Using the identity (a+b) × (a-b) = a^2 - b^2
(20)^2 - (1)^2 = 400 - 1 = 399
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