an=2+(n-1)*2=2n
bn=2n加 3n方是求bn=2n+3n方的Sn是(2n+3n)^2还是2n+3n^2?先按后者给你算了Sn=b1+...+bn=(1+n)n+(1/2)n(n+1)(2n+1)=n(n+1)(n+3/2)利用1+2^2+3^2+...+(n-1)^2 + n^2=n(n+1)(2n+1)/6计算.2n+3的n次方Sn=b1+...+bn=(1+n)n+(3/2)(-1+3^n)