有两个等差数列{an},{bn},满足a1+a2+…+an/(b1+b2+…+bn)=5n/(3n+6),则a7/b7=?
问题描述：
有两个等差数列{an},{bn},满足a1+a2+…+an/(b1+b2+…+bn)=5n/(3n+6),则a7/b7=?
答
13/9
an/bn=S2n-1/T2n-1
答
a1+a2+…+an/(b1+b2+…+bn)=(a1+an)/(b1+bn)
=5n/(3n+6),
所以a7/b7=2a7/2b7=（a1+a13)/(b1+b13)
=5*13/3*13+6=13/9