\(xy\left(x+y\right)+yz\left(y+z\right)+xz\left(x+z\right)+3xyz\\ =xy\left(x+y\right)+xyz+yz\left(y+z\right)+xyz+xz\left(x+z\right)+xyz\\ =xy\left(x+y+z\right)+yz\left(x+y+z\right)+xz\left(x+y+z\right)\\ =\left(x+y+z\right)\left(xy+yz+xz\right)\)
\(xy\left(x+y\right)+yz\left(y+z\right)+xz\left(x+z\right)+3xyz\)
\(=\left[xy\left(x+y\right)+xyz\right]+\left[yz\left(y+z\right)+xyz\right]+\left[xz\left(x+z\right)+xyz\right]\)
\(=xy\left(x+y+z\right)+yz\left(x+y+z\right)+xz\left(x+y+z\right)\)
\(=\left(xy+yz+zx\right)\left(x+y+z\right)\)
\(xy\left(x+y\right)+yz\left(y+z\right)+xz\left(x+z\right)+3xyz\\ =\left[xy\left(x+y\right)+xyz\right]+\left[yz\left(y+z\right)+xyz\right]+\left[xz\left(x+z\right)+xyz\right]\\ =xy\left(x+y+z\right)+yz\left(x+y+z\right)+xz\left(x+z+y\right)\\ =\left(x+y+z\right)\left(xy+yz+xz\right)\)
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