cosx+cosy+cosz=1+4sin(x/2)sin(y/2)sin(z/2)
cosx+cosy=1-cosz+4sin(x/2)sin(y/2)sin(z/2)
2cos[(x+y)/2]cos[(x-y)/2]
=2sin^(z/2) -2sin(z/2)[cos[(x+y)/2]-cos[(x-y)/2]
[cos(x+y)/2 +sin(z/2)][cos(x+y)/2 -sin(z/2)]=0
XYZ都是锐角
==>cos(x+y)/2 -sin(z/2)]=0
==>x+y+z=π