数列an的通项公式为an=2n+1,bn=1/

a1+a2+a3...+an=n(n+2)bn=1/(n(n+2))=(1/2)(n+2-n)/(n(n+2))=(1/2)(1/n-1/(n+2))Sbn=(1/2)(1/1-1/3+1/2-1/4+1/3-1/5+.+1/n-1/(n+2))=(1/2)(1+1/2-1/(n+1)-1/(n+2))=(3n^2+5n)/(4n^2+12n+8)