两边同时求导根据链式法则1/2（x²+y²）’/(x²+y²)=(x/y)'/[1+(x/y)²]1/2(2x+2yy')/(x²+y²)=（y-xy’）/【x²+y²】化简得x+yy'=y-xy'y'=（y-x）/(x+y)...