z(x,y)=(x+y)/(x^2+y^2)+i(x-y)/(x^2+y^2)
令：u=(x+y)/(x^2+y^2)
v=(x-y)/(x^2+y^2)
z=u+iv
z'x=u'x+iv'x
u'x=(y^2-x^2-2xy)/(x^2+y^2)^2
v'x=(y^2-x^2+2xy)/(x^2+y^2)^2
z'x=(y^2-x^2-2xy)/(x^2+y^2)^2 +i(y^2-x^2+2xy)/(x^2+y^2)^2
z'y、z''xx、z''xy、z''xy 可用类似方法算出.