g(x^3-1)-g(0)=x y=x^3-1,得x=（y+1）^(1/3) 所以,g（y）=（y+1）^(1/3)+g(0) f(y)=g'(y)=1/3（y+1）^(-2/3) 7带入.f(7)=1/12
