Sn是等差数列前n项和,a5/a6=5/9,S11/S5=
问题描述：
Sn是等差数列前n项和,a5/a6=5/9,S11/S5=
答
a5=a1+4d
a6=a1+5d
a5/a6=(a1+4d)/(a1+5d)=5/9
5*a1+25d=9*a1+36d
a1=-11/4*d
S11=(a1+a11)*11/2=(-11/2*d+10*d)*11/2=99/4*d
S5=(a1+a5)*5/2=(-11/2*d+4*d)*5/2=-15/4*d
S11/S5=(99/4*d)/(-15/4*d)=-33/5