z=arctan(x+y)/(x-y)的全微分
问题描述：
z=arctan(x+y)/(x-y)的全微分
答
(x+y/x-y)' *[1/1+(x+y/x-y)^2]按x是未知量时，是dx，y是未知量时，是dy，
答
(-ydx+xdy)/(x^2+y^2)
答
z=arctan(x+y)/(x-y)
z'x=[(1+y')/(1+(x+y)^2)] /(x-y) +arctan(x+y)(1-y')/(x-y)^2
z'y=[(1+x')/(1+(x+y)^2]/(x-y)+arctan(x+y)(x'-1)/(x-y)^2
dz=z'x dx+ z'y dy