(a+1)^2+(a^2+1)^2+(a^3+1)^2+…（a^n+1）^2
问题描述：
(a+1)^2+(a^2+1)^2+(a^3+1)^2+…（a^n+1）^2
答
bn=(a^n+1)^2=a^(2n)+2a^n+1
Sn=b1+...+bn=a^2[a^(2n)-1]/(a^2-1)+2a(a^n-1)/(a-1)+n