答：Sn=1/1*4+1/4*7+..+1/(3n-2)(3n+1)=(1-1/4)*1/3+(1/4-1/7)*1/3+...+(1/(3n-2)-1/(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=1/3-1/(9n+3)