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If f x = 3 x + 6 , g x = 4 x + k and f o g x = g o f x then k =

Options

  1. A-9
  2. B18
  3. C1 9
  4. D9

Correct answer

D. 9

Step-by-step solution

We have, f x = 3 x + 6 , g x = 4 x + k Since, f o g x = g o f x ⇒ f g x = g f x ⇒ f 4 x + k = g 3 x + 6 ⇒ 3 4 x + k + 6 = 4 3 x + 6 + K ⇒ 3 k - k = 24 - 6 ⇒ 2 k = 18 ⇒ k = 9

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