1/(3n-2)×(3n+1)=1/3*[1/(3n-2)-1/(3n+1)]
故:1/1×4,1/4×7,1/7×10,1/(3n-2)×(3n+1)的前n项和是:
1/3[1-1/4+1/4-1/7+1/7-1/10+...+1/(3n-2)-1/(3n+1)]
=1/3[1-1/(3n+1)]
=n/(3n+1)
选 B
1/(3n-2)×(3n+1)=1/3*[1/(3n-2)-1/(3n+1)]
故:1/1×4,1/4×7,1/7×10,1/(3n-2)×(3n+1)的前n项和是:
1/3[1-1/4+1/4-1/7+1/7-1/10+...+1/(3n-2)-1/(3n+1)]
=1/3[1-1/(3n+1)]
=n/(3n+1)
选 B