Jika g(x) = x² - 1 dan fungsi (f•g)(x) = x⁴ maka f(4) adalah

(f o g)(x) = f(g(x)) = x^4

f(x^2 - 1) = x^4

Buat 2 permisalan
x^2 - 1 = a ...... (1)
x^2 = a + 1
x = √(a + 1) ..... (2)

Maka :
f(a) = ( (√a + 1) )^4
f(a) = ((a + 1)^ (1/2) )^4
f(a) = (a + 1)^2

Dit. f(4)
f(4)
= (4 + 1)^2
= 5^2
= 25