a8/b8=(a1+a15)/(b1+b15).等差中项
S15=15*(a1+a15)/2
T15=15*(b1+b15)/2
a8/b8=S15/T15
=2*15/3*15+1
=15/23
等差数列（an）,（bn）的前n项和为Sn,Tn,且Sn/Tn=2n/3n+1,则a8/b8=?
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