Đề sai r bạn phải là xy(x+y)+yz(y+z)+zx(z+x)+2xyz chứ
\(xy\left(x+y\right)+yz\left(y+z\right)+zx\left(z+x\right)+3xyz\)
\(=xy\left(x+y\right)+xyz+yz\left(y+z\right)+xyz+zx\left(z+x\right)+xyz\)
\(=xy\left(x+y+z\right)+yz\left(x+y+z\right)+zx\left(x+y+z\right)\)
\(=\left(xy+yz+zx\right)\left(x+y+z\right)\)