If g(x) = 3x2 + 2x - 3, f(0) = -3 and 4g(f(x)) = 3x2 - 32x + 72, then f(g(2)) is equal to:
- 72
- - 256
- 256
- - 72
Answer
(1) 72
Solution
4g(f(x)) = 12f(x)2 + 8f(x) - 12 = 3x2 - 32x + 72
Degree 2 on the right f is linear: let f(x) = ax + b; f(0) = -3 b = -3
12(ax - 3)2 + 8(ax - 3) - 12
= 12a2x2 - 64ax + 72
Comparing with 3x2 - 32x + 72:
12a2 = 3 and 64a = 32 a = 12
f(x) = x2 - 3
g(2) = 12 + 4 - 3 = 13
f(g(2)) = f(13) = 132 - 3 = 72
Answer: (1)