Multiply $(7 a - 9 b)$ and $(7 a - 9 b)$ .

49 a^{2} - 126 a b + 81 b^{2}

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49 a^{2} - 124 a b + 81 b^{2}

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49 a^{2} - 122 a b + 81 b^{2}

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49 a^{2} - 120 a b + 81 b^{2}

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Solution

The correct option is B $49 a^{2} - 126 a b + 81 b^{2}$
$(7 a - 9 b) (7 a - 9 b)$
Using, $(a - b)^{2} = a^{2} - 2 a b + b^{2}$
$= (7 a)^{2} - 2.7 a .9 b + (9 b)^{2}$
$= 49 a^{2} - 126 a b + 81 b^{2}$

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25 (2 a - b)^{2} - 81 b^{2}

Factorise:

$25 (2 a - b)^{2} - 81 b^{2}$

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