两边对x求导1/[1+(y/x)^2]×(y/x)'=1/2×1/(x^2+y^2)×(x^2+y^2)'，也就是：x^2/(x^2+y^2)×(xy'-y)/x^2=1/2×1/(x^2+y^2)×(2x+2yy')化简得：y'=(2x+y)/(x-2y)
