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

问题描述:

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,

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