两边同时对x求导,得
(2x+2yy')/(x²＋y²)=1/(1+y²/x²)· （xy'-y）/x²
(2x+2yy')/(x²＋y²)=1/(x²+y²)· （xy'-y）
2x+2yy'=xy'-y
(x-2y)y'=2x+y
y'=(2x+y)/(x-2y)