an=1/(1+2+3+……+n)=2/(n+1)(n)=2(1/n-1/(n+1))Sn=a1+a2+...an=2[(1/1-1/2)+(1/2-1/3)+...+(1/n-1/(n+1)]=2[1/1-1/(n+1)]=2n/(n+1)
