Functions help

Gsharma90

I have been stuck on this question for quite some time. Any help and advice is appreciated :smile:

f(x) = 2x +c
g(x) = cx + 4
fg(x) = 8x + d

c and d are constants

work out the value of d

Reply 1

Prasiortle

Original post by Gsharma90

I have been stuck on this question for quite some time. Any help and advice is appreciated :smile:

f(x) = 2x +c
g(x) = cx + 4
fg(x) = 8x + d

c and d are constants

work out the value of d

We have fg(x) = f(cx+4) = 2(cx+4)+c = 2cx + 8 + c. Now we need this to be identically equal to (8x+d), so comparing x coefficients gives 2c = 8 and thus c = 4, and next, comparing constant coefficients gives 8 + c = d, so d = 8+4 = 12.

Reply 2

mqb2766

You've got two unknowns (c & d) and two equations (for the "x" coefficient and the constant term).

fg(x) = 2(cx+4) + c = 8x + d

and solve from there?