设使等差数列的前n项和,若a5/a9=5/9,则S9/S5=?A.AB.-1C.2D.1/2设使等差数列的前n项和，若a5/a3=5/9,则S9/S5=?A.1B.-1C.2D.1/2抱歉打错题了这是正确的

Question

设使等差数列的前n项和,若a5/a9=5/9,则S9/S5=?
A.A B.-1 C.2 D.1/2
设使等差数列的前n项和，若a5/a3=5/9,则S9/S5=?
A.1 B.-1 C.2 D.1/2
抱歉 打错题了 这是正确的

Answer 1

a5/a3 = a5/(a5-2d) = 5/9
=>
d =-2a5/5
=>
S5 = 5a5-10d = 9a5
S9 = 9a5
=>
S9/S5 = 9a5/9a5 = 1
选 A