∂z/∂x=1/(1+y²/x²)*(-y/x²)=-y/(x²+y²) ∂z/∂y=1/(1+y²/x²)*1/x=x/(x²+y²) ∂²z/∂x²=y/(x²+y²)*2x=2xy/(x²+y²)² ∂²z/∂x∂y=-[x²+y²-2y²]/(x²+y²)²=(y²-x²)/(x²+y²)² ∂²z/∂y²=-2xy/(x²+y²)²
