x7y+x5y+x3y=x3y.(x4+x2+1)
=x3y.(x4+2x2+1-x2)
=x3y.[(x2+1)2-x2]
=x3y.(x2-x+1)(x2+x+1)
\(=x^3y\left(x^4+x^2+1\right)\)
\(=x^3y\left(x^4+2x^2+1-x^2\right)\)
\(=x^3y\left[\left(x^2+1\right)-x^2\right]\)
\(=x^3y\left(x^2-x+1\right)\left(x^2+x+1\right)\)