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