(完整版)裂项相消法专项高考真题训练
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裂项相消法专题
1.(2014?成都模拟)等比数列{a n }的各项均为正数,且2a 1+3a 2=1,a 32=9a 2a 6, (Ⅰ)求数列{a n }的通项公式;
(Ⅱ)设b n =log 3a 1+log 3a 2+…+log 3a n ,求数列{
}的前n 项和.
【答案】(Ⅰ)设数列{a n }的公比为q ,由a 32=9a 2a 6有a 32=9a 42,∴q 2=. 由条件可知各项均为正数,故q=. 由2a 1+3a 2=1有2a 1+3a 1q=1,∴a 1=. 故数列{a n }的通项式为a n =.
(Ⅱ)b n =+
+…+=﹣(1+2+…+n)=﹣,
故=﹣=﹣2(﹣
)
则
+
+…+
=﹣2[(1﹣)+(﹣)+…+(﹣
)]=﹣
, ∴数列{}的前n 项和为﹣
.,
2,(2013?江西)正项数列{a n }满足﹣(2n ﹣1)a n ﹣2n=0.
(1)求数列{a n }的通项公式a n ; (2)令b n =
,求数列{b n }的前n 项和T n .
【答案】(1)由正项数列{a n }满足:﹣(2n ﹣1)a n ﹣2n=0,
可有(a n ﹣2n )(a n +1)=0 ∴a n =2n . (2)∵a n =2n ,b n =,∴b n =
= =,
T n =
=
=
.
数列{b n }的前n 项和T n 为.
3.(2013?山东)设等差数列{a
n }的前n项和为S
n
,且S
4
=4S
2
,a
2n
=2a
n
+1.
(Ⅰ)求数列{a
n
}的通项公式;
(Ⅱ)设数列{b
n }满足=1﹣,n∈N*,求{b
n
}的前n项和T
n
.
【答案】(Ⅰ)设等差数列{a
n }的首项为a
1
,公差为d,由S
4
=4S
2
,a
2n
=2a
n
+1有:,
解有a
1
=1,d=2.
∴a
n
=2n﹣1,n∈N*.
(Ⅱ)由已知++…+=1﹣,n∈N*,有:
当n=1时,=,
当n≥2时,=(1﹣)﹣(1﹣)=,∴,n=1时符合.∴=,n∈N*
由(Ⅰ)知,a
n
=2n﹣1,n∈N*.
∴b
n
=,n∈N*.
又T
n
=+++…+,
∴T
n
=++…++,
两式相减有:T
n
=+(++…+)﹣
=﹣﹣
∴T
n
=3﹣.
4.(2010?山东)已知等差数列{a
n }满足:a
3
=7,a
5
+a
7
=26.{a
n
}的前n项和为S
n
.
(Ⅰ)求a
n 及S
n
;
(Ⅱ)令(n∈N*),求数列{b
n }的前n项和T
n
.
【答案】(Ⅰ)设等差数列{a
n
}的公差为d,
∵a
3=7,a
5
+a
7
=26,
∴有,
解有a
1
=3,d=2,
∴a
n
=3+2(n﹣1)=2n+1;
S
n
==n2+2n;
(Ⅱ)由(Ⅰ)知a
n
=2n+1,
∴b
n
====,
∴T
n
===,
即数列{b
n }的前n项和T
n
=.
5.(2008?四川)在数列{a
n }中,a
1
=1,.
(Ⅰ)求{a
n
}的通项公式;
(Ⅱ)令,求数列{b
n }的前n项和S
n
;
(Ⅲ)求数列{a n}的前n项和T n.
【答案】(Ⅰ)由条件有,又n=1时,,
故数列构成首项为1,公式为的等比数列.∴,即.
(Ⅱ)由有,,两式相减,有:,∴.
(Ⅲ)由有.
∴T
n =2S
n
+2a
1
﹣2a
n+1
=.
6.(2010?四川)已知等差数列{a
n
}的前3项和为6,前8项和为﹣4.
(Ⅰ)求数列{a
n
}的通项公式;
(Ⅱ)设b
n =(4﹣a
n
)q n﹣1(q≠0,n∈N*),求数列{b
n
}的前n项和S
n
.
【答案】(1)设{a
n
}的公差为d,由已知有
解有a
1
=3,d=﹣1
故a
n
=3+(n﹣1)(﹣1)=4﹣n;
(2)由(1)的解答有,b
n
=n?q n﹣1,于是
S
n
=1?q0+2?q1+3?q2+…+n?q n﹣1.
若q≠1,将上式两边同乘以q,有
qS
n
=1?q1+2?q2+3?q3+…+n?q n.
上面两式相减,有
(q﹣1)S
n
=nq n﹣(1+q+q2+…+q n﹣1)
=nq n﹣
于是S
n
=
若q=1,则S
n
=1+2+3+…+n=
∴,S
n
=.
7.(2010?四川)已知数列{a
n }满足a
1
=0,a
2
=2,且对任意m、n∈N*都有a
2m﹣1
+a
2n﹣1
=2a
m+n﹣1
+2(m
﹣n)2
(1)求a
3,a
5
;
(2)设b
n =a
2n+1
﹣a
2n﹣1
(n∈N*),证明:{b
n
}是等差数列;
(3)设c
n =(a
n+1
﹣a
n
)q n﹣1(q≠0,n∈N*),求数列{c
n
}的前n项和S
n
.
【答案】(1)由题意,令m=2,n=1,可有a
3=2a
2
﹣a
1
+2=6
再令m=3,n=1,可有a
5=2a
3
﹣a
1
+8=20
(2)当n∈N*时,由已知(以n+2代替m)可有
a 2n+3+a
2n﹣1
=2a
2n+1
+8
于是[a
2(n+1)+1﹣a
2(n+1)﹣1
]﹣(a
2n+1
﹣a
2n﹣1
)=8
即b
n+1﹣b
n
=8
∴{b
n
}是公差为8的等差数列
(3)由(1)(2)解答可知{b
n }是首项为b
1
=a
3
﹣a
1
=6,公差为8的等差数列
则b
n =8n﹣2,即a
2n+1
﹣a
2n﹣1
=8n﹣2
另由已知(令m=1)可有
a
n
=﹣(n﹣1)2.
∴a
n+1﹣a
n
=﹣2n+1=﹣2n+1=2n
于是c
n
=2nq n﹣1.
当q=1时,S
n
=2+4+6++2n=n(n+1)
当q≠1时,S
n
=2?q0+4?q1+6?q2+…+2n?q n﹣1.两边同乘以q,可有
qS
n
=2?q1+4?q2+6?q3+…+2n?q n.
上述两式相减,有
(1﹣q)S
n
=2(1+q+q2+…+q n﹣1)﹣2nq n
=2?﹣2nq n
=2?
∴S
n
=2?
综上所述,S
n
=.
8.(2009?湖北)已知数列{a
n }是一个公差大于0的等差数列,且满足a
3
a
6
=55,a
2
+a
7
=16
1)求数列{a
n
}的通项公式;
2)数列{a
n }和数列{b
n
}满足等式a
n
=(n∈N*),求数列{b
n
}的前n项和S
n
.
【答案】(1)设等差数列{a
n
}的公差为d,
则依题意可知d>0由a
2+a
7
=16,
有,2a
1
+7d=16①
由a
3a
6
=55,有(a
1
+2d)(a
1
+5d)=55②
由①②联立方程求,有
d=2,a
1=1/d=﹣2,a
1
=(排除)
∴a
n
=1+(n﹣1)?2=2n﹣1
(2)令c
n =,则有a
n
=c
1
+c
2
+…+c
n
a
n+1
=c
1
+c
2
+…+c
n+1
两式相减,有
a n+1﹣a
n
=c
n+1
,由(1)有a
1
=1,a
n+1
﹣a
n
=2
∴c
n+1=2,即c
n
=2(n≥2),
即当n≥2时,
b n =2n+1,又当n=1时,b
1
=2a
1
=2
∴b
n
=
于是S
n =b
1
+b
2
+b
3
+…+b
n
=2+23+24+…2n+1=2n+2﹣6,n≥2,
.