Find the square of 21 by using the formula $(a + b)^{2} = a^{2} + 2 a b^{2} + b^{2}$

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Solution

We know that $(a + b)^{2} = a^{2} + 2 a b^{2} + b^{2}$
$(21)^{2} = (20 + 1)^{2} = (20)^{2} + 2 (20) (1) + (1)^{2}$
$= 400 + 40 + 1 = 441$

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