已知两个等比数列{an} {bn},其前n项和分别为Sn和Tn,并且Sn/Tn=(7n+2)/(n+3),则a7/b8=

问题描述:

已知两个等比数列{an} {bn},其前n项和分别为Sn和Tn,并且Sn/Tn=(7n+2)/(n+3),则a7/b8=
详解

Sn/Tn=(a1+an)/(b1+bn)=(nd1+2a1-d1)/(nd2+2b1-d2)=(7n+2)/(n+3)
令d2=m,m≠0,则d1=7m,2a1-d1=2m,2b1-d2=3m
得a1=(9/2)m,b1=2m
a7=(9/2)m+42m=(93/2)m
b8=2m+7m=9m
a7/b8=93/18=31/6
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