Use suitable identities to find the following product: (3x + 4) (3x – 5)

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Solution

Using identity, (x + a) (x + b) = $x^{2}$ + (a+b)x + ab

Here x = 3x, a = 4 and b = -5

(3x + 4) (3x – 5) = ${(3 x)}^{2}$ + (4+(-5))(3x) + (4)(-5)

= 9 $x^{2}$ + (4-5)(3x) + (-20)
= 9 $x^{2}$ -3x -20

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