1／1×4 ＋ 1／4×7 ＋ 1／7×10 ＋……＋1／（3n-2）×（3n+1)
=1/3(1-1/4+1/4-1/7+.+1/(3n-2)-1/(3n+1))
=1/3(1-1/(3n+1))
=1/3*3n/(3n+1)
=n/(3n+1)