等差数列{an}中,S5=28,S10=36 (Sn为前n项和),则S15等于?

学习时间:2026-04-04 19:49:10 阅读：7715

等差数列{an}中,S5=28,S10=36 (Sn为前n项和),则S15等于?我算出来是24，

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2026-04-04 19:49:10

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s5
=5a1+10d=28
s10
=10a1+45d=36
10a1+20d=56
10a1+45d=36
25d=-20
得到 d=-4/5
a1=36/5
s15
=15a1+105d
=108-84
=24
答案错了