x^5-1=x^5-x^4+x^4-x^3+x^3-x^2+x^2-x+x-1=x^4(x-1)+x^3(x-1)+x^2(x-1)+x(x-1)+(x-1)=(x-1)(x^4+x^3+x^2+x+1) 由此可知(x-1)(1+x+x^2+x^3+…+x^n)=x^(n+1)-1
