z=x+yi
则(z-1)/(z+1)
=[(x-1)+yi][(x+1)-yi]/[(x+1)-yi][(x+1)+yi]
=[(x²-1+y²)+2yi]/(x²-2x+1+y²)
纯虚数则x²-1+y²=0,2y≠0
所以x²+y²=1,不包括(±1,0)