f'x(x,y)=1/(1+y²/x²) *(y/x)'=1/(1+y²/x²)(-y/x²)=-y/(x²+y²)f'y(x, y)=1/(1+y²/x²)*(y/x)'=1/(1+y²/x²)(1/x)=x/(x²+y²)f'x(1,1)=-1/2f'y(-1, -1)=-1/2
