将等式乘以x^2得：
3n*x^2=x^2+2x^4+3x^6+…+9x^18+10x^20
两个等式相减得：
3n-3n*x^2=1+x^2+x^4+x^6+…+x^18-10x^20
3n-3n*x^2=1*（1-x^20)/(1-x^2)-10x^20