已知{an},{bn}均为等差数列,他们的前n项和分别为Sn,Tn,且Sn/Tn=(3n-1)/(n+1),求a7/b7的值

Question

已知{an} ,{bn}均为等差数列,他们的前n项和分别为Sn ,Tn ,且Sn/Tn =(3n-1)/(n+1) ,求a7/b7的值

Answer 1

7+7=1+13
2a7=a1+a13
2b7=b1+b13
所以a7/b7=(a1+a13)/(b1+b13)
=[[(a1+a13)*13/2]/[(b1+b13)*13/2]
=S13/T13
=(3*13-1)/(13+1)
=19/7