(x^21+x^15+x^12+x^9+x^6+x^3+1)/(x^2+x+1)

=x^18+x^15+x^12+x^9+x^6+x^3+1+x^3-1+5-4
分解因式
把下面的18换成21就是了
=(x^18+1)+(x^15+1)+(x^12+1)+(x^9+1)+(x^6+1)+(x^3+1)+(x^3-1)-4
=(x^18+1)+(x^15+1)+(x^12+1)+(x^9+1)+(x^6+1)+(x^6-1)-4
=(x^18+1)+(x^15+1)+(x^12+1)+(x^9+1)+(x^9-1)-4
=(x^18+1)+(x^15+1)+(x^12+1)+(x^12-1)-4
=(x^18+1)+(x^15+1)+(x^15-1)-4
=(x^18+1)+(x^18-1)-4
=x^36-5