对x求导1/√(x²+y²)*[1/2√(x²+y²)]*(2x+2y*y')=1/(1+y²/x²)]*(y'*x-y)/x²(x+y*y')/(x²+y²)=(y'*x-y)/(x²+y²)x+y*y'=y'*x-y(x-y)*y'=x+yy'=(x+y)/(x-y)...