an=2n-1Sn = a1+a2+...+an = n^21/Sn = 1/n^2for n>=2,1/Sn < 1/[n(n-1)] = 1/(n-1) -1/n1/S2+1/S3+...+1/Sn < 1 -1/n1/S1+1/S2+...+1/Sn=1 +[1/S2+...+1/Sn]< 1 + (1- 1/n)