MCQ
If $f(x) = ax + b$ and $g(x) = cx + d$, then $f(g(x)) = g(f(x))$ is equivalent to
  • A
    $f(a) = g(c)$
  • B
    $f(b) = g(b)$
  • $f(d) = g(b)$
  • D
    $f(c) = g(a)$

Answer

Correct option: C.
$f(d) = g(b)$
c
(c) We have $f(x) = ax + b,\,g(x) = cx + d$ and $f(g(x)) = g(f(x))$

==> $f(cx + d) = g(ax + b)$ 

==> $a[cx + d] + b = c[ax + b] + d$

==> $ad + b = cb + d$==> $f(d) = g(b)$.

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