Question 85 (vii)

Expand the following, using suitable identities.

$(2 x + 9) (2 x - 7)$

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Solution

We have,
$(2 x + 9) (2 x - 7) = (2 x + 9) [2 x + (- 7)] = (2 x)^{2} + [9 + (- 7)] 2 x + 9 \times (- 7)$
[using the identity, $(x + a) (x + b) = x^{2} + (a + b) x + a b$ ]
$= 4 x^{2} + 4 x - 63$

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