x+x²+x³+x^4+x^5+x^6+x^7+x^8
=(x+x²+x³+x^4)+(x^5+x^6+x^7+x^8)
=x(1+x+x²+x³)+x^5(1+x+x²+x³)
=x×0+x^5×0
=0+0
=0