等差数列前n项求和设Sn是等差数列{an}的前n项和,若a5\a3=5\9,则S9\S5=()A.1 B.-1 C.2 D.1\2求详解啊～谢谢

等差数列前n项求和设Sn是等差数列{an}的前n项和,若a5\a3=5\9,则S9\S5=()A.1 B.-1 C.2 D.1\2求详解啊～谢谢
等差数列前n项求和
设Sn是等差数列{an}的前n项和,若a5\a3=5\9,则S9\S5=()
A.1 B.-1 C.2 D.1\2
求详解啊～谢谢

等差数列前n项求和设Sn是等差数列{an}的前n项和,若a5\a3=5\9,则S9\S5=()A.1 B.-1 C.2 D.1\2求详解啊～谢谢
S9/S5=((a1+a9)*9)/((a1+a5)*5)=(2*a5*9)/(2*a3*5)=9*a5/5*a3=1
关键2*a5=a1+a9 2*a3=a1+a5

S9/S5=(9a5)/(5a3)=1
选A