Use the product (a + b) (a – b) = a^2 – b^2 to evaluate : 21 19 - Vidyadip

Question

Use the product $ (a + b) (a – b) = a^2 – b^2$ to evaluate : $21 \times 19$

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Answer

$21 \times 19 $
$= (20+1) (20−1)$
$= (20)^2 − (1)^2$
$= 400 − 1$
$= 399$

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