g(x) = x^n * f(x)h(x) = x^n由柯西中值定理，存在s使：（那两个同时取s即可）1= (g(b)-g(a))/(f(b)-f(a)) =g'(s)/h'(s)=( n*s^(n-1)*f(s) + s^n*f'(s)) / (n*s^(n-1))= (f(s)+s*f'(s)/n)*(s/s)^(n-1)
