an=3*n sn=(3+3n)*n/2,1/Sn=(2/3)*(n*(n+1))=(2/3)*(1/n-1/(n+1)) 1/S1+1/S2+1/S3+…+1/Sn=2/3*
(1-1/2+1/2-1/3+.+1/n-1/(n+1))=(2/3)*(1-1/(n+1))