$x^{7}$+$x^{5}$+1
=($x^{7}$-x)+($x^{5}$-$x^{2}$)+($x^{2}$+x+1)
=x($x^{6}$-1)+$x^{2}$($x^{3}$-1)+($x^{2}$+x+1)
=x($x^{3}$-1)($x^{3}$+1)+$x^{2}$(x-1)($x^{2}$+x+1)+($x^{2}$+x+1)
=($x^{2}$+x+1)[x(x-1)($x^{2}$+1)+$x^{2}$(x-1)+1]
=($x^{2}$+x+1)($x^{5}$-$x^{4}$+$x^{3}$-x+1)
(Học tốt nhé!)