设z=ln√x2+y2求∂2z/∂x2+∂2z/∂y2.
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
设z=ln√x2+y2求∂2z/∂x2+∂2z/∂y2.
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