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If f(x) = 3x + 6 ,g(x) = 4x + k and fog...

If `f(x) = 3x + 6 ,g(x) = 4x + k` and `fog(x) = gof(x) ` then k …..

A

`-9`

B

18

C

`(1)/(9)`

D

9

Text Solution

Verified by Experts

The correct Answer is:
D
We have,
`f(x) = 3x + 6, g(x) = 4x + k`
Since, for (X) = gof (x)
`implies f(g) (x) = g(f(x))`
`implies f(4x + k) = g(3x + 6)`
`implies 3(4x + k) + 6 = 4 (3x + 6) + k`
`implies 3k - k = 24 - 6`
`implies 2k = 18`
`implies k = 9`
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