设z=(x-y)/(x+y),求∂2z/∂x∂y
∂z/∂x=[(x+y)-(x-y)]/(x+y)2=2y/(x+y)2,∂2z/∂x∂y=[2(x+y)2-2(x+y)•2y]/(x+y)4=2(x2-y2)/(x+y)4=2(x-y)]/(x+y)3
设z=(x-y)/(x+y),求∂2z/∂x∂y
∂z/∂x=[(x+y)-(x-y)]/(x+y)2=2y/(x+y)2,∂2z/∂x∂y=[2(x+y)2-2(x+y)•2y]/(x+y)4=2(x2-y2)/(x+y)4=2(x-y)]/(x+y)3