用an的后一项减去前一项得2，所以an是等差数列。所以San=a1+a2+a3+...+an=(3+2n+1)*n/2=(n+2)*nbn=San/n=n+2类似an,bn也是等差数列。Sbn=（3+n+2）*n/2=(n+5)*n/2
