y'=f'(u)=[f(g(x))]'=dy/du*g'(x),先是外层函数对u求导，再是里层函数对x求导。也就是可以归纳成公式y'=f(u）’u(x)'
y'=f'(u)=[f(g(x))]'=dy/du*g'(x),先是外层函数对u求导，再是里层函数对x求导
