u'(y)=1/f'(x)=1/f'(u(y))u''(y)=(1/f'(u(y)))'=-1/(f'(x))^2 * f''(x) * u‘(y) （复合函数求导）=-f''(x)/(f'(x))^2 * 1/f'(x)=-f''(x)/(f'(x))^3
