Z=f'x(x,y)=xy*[x^(xy-1)]*y
Z=f'y(x,y)=xy*[x^(xy-1)]*x
自变量在指数上的，简单的方法是取对数，相信见到过这种方法奥
z = x^(x y)
lnz = xylnx
(1/z)f'x(z) = xy(1/x) + (lnx) (y)
= y+ylnx
f'x(z) = (1+lnx)z = y(1+lnx)(x^(xy))
lnz = xylnx
(1/z) f'y(z) = xlnx
f'y(z) = z(xlnx) =(z^(xy) ) xlnx
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