g(x)=x^2+1 dan (fog)(x)=2x^2-1, maka f(x)=?

f(g(x))= 2x^2 - 1
g(x)=x^2 +1
f(g(x))= 2 g(x)^2 -1
= 2 (x^2 + 1)^2 -1
= 2 (x^4 + 2x^2 + 1) -1
= 2 x^4 + 4 X^2 + 2 -1
f(x)= 2x^4 + 4x^2 + 1.