等差数列{an}和{bn}的前n项和分别是Sn,Tn,且a5/b5=2/3,则S9/T9=?
问题描述:
等差数列{an}和{bn}的前n项和分别是Sn,Tn,且a5/b5=2/3,则S9/T9=?
答
S9/T9
=[(a1+a9)*9/2]/[(b1+b9)*9/2]
=(a1+a9)/(b1+b9)
=(2a5)/(2b5)
=a5/b5
=2/3