求函数z=(x+y)/(x-y)的全微分．
【正确答案】：因∂z/∂x=[(x-y)-(x+y)]/(x-y) 2=-2y/(x-y) 2 ∂z/∂y=[(x-y)+(x+y)]/(x-y)2=2x/(x-y)2 且它们都是连续的，故由全微分公式得 dz=[-2y/(x-y)2]dx+[2x/(x-y)2]dy =[2/(x-y)2](xdy-ydx)
求函数z=(x+y)/(x-y)的全微分．
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