an=1/2*[1/n - 2/(n+1) +1/(n+2)]Sn=1/2{(1/1 -2/2 + 1/3)+(1/2 - 2/3 +1/4)+...+ [1/n - 2/(n+1) +1/(n+2)]} =1/2[1/1 -1/2 - 1/(n+1) +1/(n+2)] =1/4-1/[2*(n+1)(n+2)]
