求z=xy/(x-y)的全微分, 我是怎麼都做不出和答案一樣的了.為什麼呢,那位親幫我做下詳細過程吧,

求z=xy/(x-y)的全微分, 我是怎麼都做不出和答案一樣的了.為什麼呢,那位親幫我做下詳細過程吧,

z=xy/(x-y)dz=f'x(x,y)dx+f'y(x,y)dyxy/(x-y)對x取偏導得[y(x-y)-xy]/(x-y)²dxxy/(x-y)對y取偏導得[x(x-y)+xy]/(x-y)²dy所以dz=[y(x-y)-xy]/(x-y)²dx+[x(x-y)+xy]/(x-y)²dy dz=-y²/(x-y)&s...