企源知识库

设z=ln√x²+y²求∂²z/∂x²+∂²z/∂y²．
【正确答案】：z=(1/2)ln(x²+y²),∂z/∂x=x/(x²+y²)，∂z/∂y=y/x(²+y²)∂²z/∂x²=y²-x²/(x²+y²)²，∂²z/∂y²=x²-y²/(x²+y²)²，∂²z/∂x²+∂²z/∂y²=0