u=(x+y)/(1-xy)z'x=u'x/(1+u²)=(1-xy-(x+y)(-y))/(1-xy)²(1+u²)=(1+y²)/((1-xy)²+(x+y)²)z'y=u'y/(1+u²)=(1-xy-(x+y)(-x))/(1-xy)²(1+u²)=(1+x²)/((1-xy)²+(x+y)²)