设y=f(x),x∈R,则：
g(x)=f[f(x)]=y^2+1=[f(x)]^2+1,
即f[f(x)]=[f(x)]^2+1,
令X=f(x)可得：f(X)=X^2+1,
∴f(x)=x^2+1,x∈R,
∴g(x)=f[f(x)]=(x^2+1)^2+1=x^4+2x^2+2,x∈R.