∂z/∂x=1/(1+x²/y²)* 1/y=y²/(y²+x²)*1/y=y/(y²+x²) ∂z/∂y=1/(1+x²/y²)*(-x/y²)=-x/(y²+x²) ∂²z/∂x²=-y/(y²+x²)²* ...
