e^(x/y)=x-y 两边同时对x求导：e^(x/y) * (y - x*y')/y²=1-y' 展开移项y*e^(x/y)-y²=x*e^(x/y)y′-y²y′=（x*e^(x/y)-y²）y′y′=【y*e^(x/y)-y²)】 /【x*e^(x/y)-y²】...