Sn=3/3{1/(1*4)+1/(2*7)+……+1/[n*(3n+1)]}
=3{1/（3*4）+1/(6*7)+……+1/[3n*(3n+1)]}
=3{1/3-1/4+1/6-1/7+1/9-1/10+……+1/3n-1/(3n+1)}
好像无解.是不是题错了?