等差数列an的前n项和为Sn,若S15为一确定常数,则下列各式也为确定常数是 A a2+a13 B a2a13 C a1+a8+a15 D a1a8a15 选哪一个 要理由

在等差数列{an},设Sn为它的前n项和,若S15>0,S16 等差数列｛an｝的前n项和为sn,若S15=90,则a8= sn为等差数列{an}的前n项的和,已知s15>0,s16 设等差数列{an}的前n项和为Sn,已知a3=11,S15*S16 设等差数列{an}的前n项和为Sn,且满足S15>0,S16 已知等差数列{an}的前n项和为Sn,S15>0,S16 等差数列{an}的前n项和为Sn,且S10-S5=3,S15=? 等差数列｛an｝的前n项和为Sn,若S3=-6 S18-S15=18 则S18等于 若等差数列an的前n项和为Sn,a8=2a3,则S15/S5= 设等差数列an的前n项和为Sn,若S6=24,S9=63,则S15= 等差数列an、s7=7.s15=75.tn为{sn/n}的前n项和,求tn、 等差数列an的前n项和为Sn.若a1=20,S10=S15,则当n=多少时,Sn最大 设等差数列{an}的前n项和为Sn ,且S15>0,a8+a90,a8+a9 设等差数列{an}的前n项和为Sn,已知a3=11,S15>0,S16 已知等差数列{an}的前n项和为sn,且a5=15,s15>0,s16 等差数列{an}的前n项和为Sn,S15=225,S9-S5=54,求通项 已知等差数列an的前n项和为sn,若是s15=30,sn=240,an-7=30,则n= 已知等差数列an的前n项和为sn,若是s15=30,sn=240,an-7=30,则n=