z=ln(x²+y²)∂z/∂x=2x/(x²+y²)∂z/∂y=2y/(x²+y²)∂²z/∂x²=[2(x²+y²)-2x·2x)]/(x²+y²)²=(2y²-2x²)/(x²+y²)²∂²z/∂x²=-2x·2y/(x²+y²)²=-4xy/(x²+y²)²∂²z/∂y²=[2(x²+y²)-2y·2y)]/(x²+y²)²=(2x²-2y²)/(x²+y²)²
