f(2) = 1/(1+2) = 1/3
g(2) = 2^2 + 2 = 6
f(g(2)) = f(6) = 1/(1+6) = 1/7
f(g(x)) = 1/(1+(x^2 + 2)) = 1 / (x^2 + 3)
x^2就是x的平方的意思