等差数列{a},{b}的前n项的和为Sn,Tn,若Sn/Tn=2n+1/3n-1 .求a7/b7,an/bn
问题描述:
等差数列{a},{b}的前n项的和为Sn,Tn,若Sn/Tn=2n+1/3n-1 .求a7/b7,an/bn
快 急
答
Sn/Tn=(2n+1)/(3n-1)所以 S(2n-1)/T(2n-1)=(4n-1)/(6n-4)又 S(2n-1)=(2n-1)[a1+a(2n-1)]/2=(2n-1)an同理 T(2n-1)=(2n-1)bn所以 an/bn=S(2n-1)/T(2n-1)=(4n-1)/(6n-4)a7/b7=27/38