(x+y+z)^2 =x²+y²+z²+2xy+2xz+2yz(x+y+z)^2=[(x+y)+z]²=(x+y)²+2(x+y)z+z²=x²+2xy+y²+2xz+2yz+z²=x²+y²+z²+2xy+2xz+2yz