等差数列{an},{bn}的前n项和分别为Sn,Tn,若Sn/Tn=2n/3n+1,则a3/b3等于多少?

题目

等差数列{an},{bn}的前n项和分别为Sn,Tn,若Sn/Tn=2n/3n+1,则a3/b3等于多少?

答案

a3/b3
=2a3/2b3
=(a1+a5)/(b1+b5)
=[(a1+a5)*5/2]/[(b1+b5)*5/2]
=S5/T5
=2*5/(3*5+1)