问题
选择题
在等差数列{an}中,设Sn为其前n项和,已知
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答案
∵
=a2 a3
,设a2=t,a3=3t1 3
∴d=a3-a2=2t,a1=a2-d=-t
∴S4=4•a1+
=8t,S5=5a1+4•3•2t 2
=15t5•4•2t 2
∴
=S4 S5
=8t 15t 8 15
故选A
在等差数列{an}中,设Sn为其前n项和,已知
|
∵
=a2 a3
,设a2=t,a3=3t1 3
∴d=a3-a2=2t,a1=a2-d=-t
∴S4=4•a1+
=8t,S5=5a1+4•3•2t 2
=15t5•4•2t 2
∴
=S4 S5
=8t 15t 8 15
故选A