{an}为等比数列,a2+a3=1,a3+a4=-2,则a5+a6+a7=( ) A.-24 B.24 C.-48 D.48
{an}为等比数列,a2+a3=1,a3+a4=-2,则a5+a6+a7=( )
A. -24
B. 24
C. -48
D. 48
数学人气:496 ℃时间:2019-12-13 22:12:52
优质解答
设等比数列{a
n}的公比为q,
则q=
=-2,
故可得a
2+a
3=a
1q+a
1q
2=2a
1=1,即a
1=
∴a
5+a
6+a
7=a
5(1+q+q
2)=
×(-2)
4(1-2+4)=24
故选B
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