Evaluate :
(5xy - 7) (7xy + 9)
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= 5xy (7xy + 9) - 7 (7xy + 9)
= 35x(1+1)y(1+1) + 45xy - 49xy - 63
= 35x2y2 - 4xy - 63
Hence, (5xy - 7) (7xy + 9) = 35x2y2 - 4xy - 63
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