左右2边取正切,左边=(X+Y)/(1-XY)=右边.
左边=arctan[(X+Y)/(1-XY)+Z]/[1-(X+Y)Z/(1-XY)]=arctanc(X+Y+Z-XYZ)/[1-XY-(X+Y)Z]=0
所以X+Y+Z-XYZ=0,既x+y+z=xyz