x7+x6+x5+x4+x3+x2+x+1x2−1x7+x6+x5+x4+x3+x2+x+1x2−1
=(x7+x6)+(x5+x4)+(x3+x2)+(x+1)(x−1)(x+1)=(x7+x6)+(x5+x4)+(x3+x2)+(x+1)(x−1)(x+1)
=x6(x+1)+x4(x+1)+x2(x+1)+(x+1)(x−1)(x+1)=x6(x+1)+x4(x+1)+x2(x+1)+(x+1)(x−1)(x+1)
=(x+1)(x6+x4+x2+1)(x−1)(x+1)=x6+x4+x2+1x−1