yz(y+z)+xz(z−x)−xy(x+y)
=−[xy(x+y)−yz(y+z)−zx(z−x)]
=−(y.[x(x+y)−z(y+z)]−zx(z−x))
=−[y.(x2+xy−zy−z2)−zx(z−x)]
=−[y.(x2−z2+xy−zy)−zx(z−x)]
=−(y.[(x+z)(x−z)+y.(x−z)]−zx(z−x))
=−[y.(x−z)(x+z+y)+zx(x−z)]
=[(x−z)[y(x+z+y)+zx]]
=−(x−z)(yx+yz+y2+zx)
=−(x−z)(yx+zx+yz+y2)
=−[(x−z)[x.(y+z)+y.(y+z)]]
=−(x−z)(y+z)(x+y)