2S9=S3+S6
2a1(1-q^9)/(1-q)=a1(1-q^3)/(1-q)+a1(1-q^6)/(1-q)
2(1-q^9)=(1-q^3)+(1-q^6)
2(1-q^3)(1+q^3+q^6)=(1-q^3)+(1-q^3)(1+q^3)
若1-q^3=0,则q=1, 是常数列,则a2=a8=a5,等差
若1-q^3≠0
2+2q^3+2q^6=2+q^3
q^3+2q^6=0
q^3=-1/2
所以a5=a2*q^3=-a2/2
a8=a2*q^6=a2/4
则a2+a5=a2/2=2a8
所以a2,a8,a5成等差数列
已知数列an是等比数列,前n项和为Sn,若S3,S9,S6成等差数列,求a2,a8,a5成等差数列
已知数列an是等比数列,前n项和为Sn,若S3,S9,S6成等差数列,求a2,a8,a5成等差数列
数学人气:235 ℃时间:2019-10-23 14:50:46
优质解答
我来回答
类似推荐
猜你喜欢
- 1甲乙合作,甲百分60,乙百分40,甲工作10天,乙工作8天,工2000元,甲乙各拿多少钱
- 2Don't worry .He is ------- to look after little Betty.A carefully enough B careful enough
- 3①We all know that J.K.Rowling is the __
- 4用“脏”英文造句,要带中文
- 5某汽车以每秒10米的速度在公路上行驶,公路正前方有一山谷,驾驶员按一声喇叭,他4秒后听到回声,这时汽车
- 6Please write the words down in your notebook.改为否定句
- 7汽车从甲地到乙地计划每小时行50千米.7小时到达,实际3小时行了180千米.这样行完全程要几小时?
- 8如图,在平行四边形ABCD中,点E.F分别在BC.AD上.且AF=CE,求证:四边形AECF是平行四边形(用平行且相等解答)
- 9已知慢车的速度是快车的5/6,两车从甲乙两站同时相向而行,在离中点4千米的地方相遇.求甲乙两站的距离是多少千米?
- 10The bottle is filled with water.(改为同义句)