Question

设S n 是等差数列{a n }的前n项和，若 a 5 a 3 = 5 9 ，则 S 9

设S n 是等差数列{a n }的前n项和，若 a 5 a 3 = 5 9 ，则 S 9 S 5 =______．

Answer 1

S 9 S 5 = ( a 1 + a 9 )×9 2 ( a 1 + a 5 )×5 2 = a 5 a 3 × 9 5 =1 故答案为1