Find the square of: $(3 a + 7 b)$ .

9 a^{2} + 12 a b + 22 b^{2}

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9 a^{2} + 2 a b - 49 b^{2}

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9 a^{2} + 42 a b + 49 b^{2}

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9 a^{2} - 42 a b + 9 b^{2}

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Solution

The correct option is D $9 a^{2} + 42 a b + 49 b^{2}$
Given, $2 a + b$ .

On squaring, we get, $(2 a + b)^{2}$ .

We know, $(x + y)^{2} = x^{2} + y^{2} + 2 x y$ .

Then,
$(2 a + b)^{2}$
$= (2 a)^{2} + b^{2} + 2 (2 a) (b)$
$= 4 a^{2} + 4 a b + b^{2}$ .

Therefore, option $D$ is correct.

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