The Student Room Group
Reply 1
For C, sub x with 4 in g(x) then sub x for your answer from g(x) into f(x)
think of it as f(g(4)) rather than fg(4).

So evaluate g(4) then use that answer as the x when evaluating f.

Do the same for d.
Reply 3
For D, sub in f(x) as x in g(x)

so

3 - 2(3x - 5)
Reply 4
Jabba1024
For D, sub in f(x) as x in g(x)

so

3 - 2(3x - 5)


thank you so much for your help! :smile:

another quick question :rolleyes:

f(x) = 5x + 4

find f^-1(x) and verify that the composite function f f ^-1(x) = x

for the first part do you just sub -x in? for the second part have nooo idea
boomboompow.
f(x) = 5x + 4

find f^-1(x) and verify that the composite function f f ^-1(x) = x

for the first part do you just sub -x in? for the second part have nooo idea


A little trick: take everything "one step back."
So make f(x)xf(x) \rightarrow x, and every xf1(x)x \rightarrow f^{-1}(x).

You'll then get:
x=5f1(x)+4x=5f^{-1}(x)+4

And then, for the first part, simply re-arrange this to make f1(x)f^{-1}(x) the subject!

It's probably best to post again once you've cracked the first part, before tackling the 2nd straight after ~
(edited 13 years ago)

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