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Using the identity, (a - b) 2 = a 2 - 2ab + b 2 we have (7a - 9b) (7a - 9b) = ( 7a -9b) 2 = (7a) 2 - 2(7a)(9b) + (9b) 2 = 49a 2 - 126ab + 81b 2

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Using the identity, (a - b)²= a²- 2ab + b²we have
(7a - 9b) (7a - 9b) = ( 7a -9b)²
= (7a)²- 2(7a)(9b) + (9b)²

= 49a²- 126ab + 81b²

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