已知Sn为等差数列{an}的前n项和,Sn=12n-n2.
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已知Sn为等差数列{an}的前n项和,Sn=12n-n2.
(1)求|a1|+|a2|+|a3|;
(2)求|a1|+|a2|+|a3|+…+|a10|;
(3)求|a1|+|a2|+|a3|+…+|an|.
(1)求|a1|+|a2|+|a3|;
(2)求|a1|+|a2|+|a3|+…+|a10|;
(3)求|a1|+|a2|+|a3|+…+|an|.
![已知Sn为等差数列{an}的前n项和,Sn=12n-n2.](/uploads/image/z/18093176-8-6.jpg?t=%E5%B7%B2%E7%9F%A5Sn%E4%B8%BA%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%EF%BC%8CSn%3D12n-n2%EF%BC%8E)
(1)∵Sn=12n-n2.∴当n=1时,a1=S1=12-1=11,
当n≥2时,an=Sn-Sn-1=(12n-n2)-12(n-1)+(n-1)2=13-2n.
当n=1时,13-2×1=11=a1,∴an=13-2n.
由an=13-2n≥0,得n≤
13
2,
∴当1≤n≤6时,an>0;当n≥7时,an<0.
∴|a1|+|a2|+|a3|=S3=12×3-32=27.
(2)|a1|+|a2|+|a3|+…+|a10|
=2S6-S10
=2(12×6-62)-(12×10-102)
=52.
(3)当1≤n≤6时,
|a1|+|a2|+|a3|+…+|an|=Sn=12n-n2,
当n≥7时,
|a1|+|a2|+|a3|+…+|an|=2S6-Sn
=n2-12n+72.
∴|a1|+|a2|+|a3|+…+|an|=
12n−n2,1≤n≤6
n2−12n+72,n≥7.
当n≥2时,an=Sn-Sn-1=(12n-n2)-12(n-1)+(n-1)2=13-2n.
当n=1时,13-2×1=11=a1,∴an=13-2n.
由an=13-2n≥0,得n≤
13
2,
∴当1≤n≤6时,an>0;当n≥7时,an<0.
∴|a1|+|a2|+|a3|=S3=12×3-32=27.
(2)|a1|+|a2|+|a3|+…+|a10|
=2S6-S10
=2(12×6-62)-(12×10-102)
=52.
(3)当1≤n≤6时,
|a1|+|a2|+|a3|+…+|an|=Sn=12n-n2,
当n≥7时,
|a1|+|a2|+|a3|+…+|an|=2S6-Sn
=n2-12n+72.
∴|a1|+|a2|+|a3|+…+|an|=
12n−n2,1≤n≤6
n2−12n+72,n≥7.
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