设Sn是等差数列(an)的前n项和,若a5除以a9等于九分之五,则S9除以S5等于?麻烦写清楚些,_数学解答

设Sn是等差数列(an)的前n项和,若a5除以a9等于九分之五,则S9除以S5等于?
麻烦写清楚些,

人气：196 ℃　时间：2020-02-03 05:17:34

解答

a5/a9 = a5/(a5+4d) = 5/9
=>
d = a5/5
=>
S5 = 5a5-10d = 3a5
S9 = 9a5
=>
S9/S5 = 9a5/3a5 = 3