1樓:澈澈
(i)設等比數列的公比為q,則
1+d=q①
1+3d=q②,
把①代入②,得d3+3d2=0,又d≠0,∴d=-3,並求得q=-2,
∴an=-3n+4,b
n=(?2)
n?1(n∈n*);
(ii)由(i)知cn=a
nbn=(?3n+4)?(?2)
n?1,sn
=c+c
+c+…+c
n=1+(?2)?(?2)+…+(?3n+4)(?2)n?1,
則?2s
n=(?2)+(?2)(?2)
+…+(?3n+7)(?2)
n?1+(?3n+4)(?2)n,
兩式相減得,3s
n=1+(?3)[(?2)+(?2)
+…+(?2)
n?1]?(?3n+4)(?2)
n=1+(?3)?2[1?(?2)
n?1]
3?(?3n+4)(?2)n,
∴sn=(n?1)(?2)
n+1.
已知等差數列{an}的公差為d(d≠0),等比數列{bn}的公比為q,a1=b1=1,a2=b2,a5=b3.(1)求數列{an}與
2樓:今生寂酒
(1)由題意可知,
1+d=q
1+4d=q
解方程可得,d=2,q=3∴an
=2n?1,b
n=n?1
(2)∵cn=an?bn=(2n-1)?3n-1∴sn=1?
1+3?31+5?32+…+(2n-1)?
3n-1∴3sn=1?3+3?32+…+(2n-3)?
3n-1+(2n-1)?3n
兩式相減可得,-2sn=1+2(3+32+…+3n-1)-(2n-1)?3n
=1+2?3(1?n?1
)1?3
-(2n-1)?3n
=1+3n-3-(2n-1)?3n=(-2n+2)?3n-2∴sn=(n?1)?n+1
已知數列{an},{bn}分別是等差、等比數列,且a1=b1=1,a2=b2,a4=b3≠b4.①求數列{an},{bn}的通項公式
3樓:薔宣朗
①設的公差為d,的公比為q,則依題意
1+d=q
1+3d=q
q≠1?
q=2d=1
∴an=1+(n-1)×1=n;
bn=1×2n-1=2n-1.(4分)
②∵sn=n(n+1)2?1
sn=2n(n+1)
=2(1n-1
n+1).
∴tn=1s+1
s+…+1sn
=2[(11?1
2)+(12?1
3)+…+(1n?1
n+1)]
=2(1-1
n+1)
=2nn+1
.(8分)
③∵cn=n?n?1
(n+1)(n+2)
2=n?n
(n+1)(n+2)
=n+1
n+2-n
n+1.
∴rn=c1+c2+…+cn=(3
?2)+(4?3
)+…+(n+1
n+2?n
n+1)
=n+1
n+2-1.
已知數列{an}為等差數列,且公差不為0,{bn}為等比數列,a1=b1=1,a2=b2,a4=b3.(ⅰ)求{an}的通項公式
若{an}是公差d≠0等差數列,{bn}是公比q≠1等比數列,已知a1=b1=1,且a2=b2,a6=b3.(1)求數列{an}和{b
4樓:回憶
(1)依題得
1+d=q
1+5d=q
?d=3
q=4,
∴an=3n-2,bn=4n-1;
(2)∵1an
an+1
=1(3n?2)(3n+1)=13
(13n?2
-13n+1
),∴sn=1
3[(11-1
4)+(14-1
7)+…+(1
3n?2
-13n+1
)]=n
3n+1
;(3)假設存在常數a,b滿足題意,把an=3n-2,bn=4n-1代入an=logabn+b,
得:3n-2=log
an?1
+b,即(3-loga4)n+(loga4-b-2)=0對一切n∈n*都成立,
∴3?log
a4=0
loga
4?b?2=0
,解得:
a=34
b=1.
即存在常數a=3
4,b=1滿足題設.
在等差數列{an},等比數列{bn}中,已知a1=b1=1,a2=b2≠1,a8=b3,(1)求數列{an}的公差d和數列{bn}的公
5樓:匿名使用者
(1)∵a1=b1=1,a2=b2≠1,a8=b3,∴1+d=q,1+7d=q2,(d≠0,q≠1)解得:d=5,q=6;
(2)由(1)知:an=1+5(n-1)=5n-4,bn=6n-1,要使對一切正整數n,都有an=logxbn+y成立,即5n-4=(n-1)logx6+y,
∴5=logx6
?4=y?logx6
,解得x=5
6y=1,∴當
x=56
y=1時對,一切正整數n,都有an=logxbn+y成立.
已知等差數列{an}的首項a1=1,公差d>0,其前n項和為sn,數列{bn}是等比數列,且b1=a2,b2=a5,b3=a14.
6樓:膽分燙
(1)由是等比數列,得b
=b?b
,即(a
+4d)
=(a+d)(a
+13d),整理得:2a
d=d.
∵a1=1,公差d>0,
∴d=2.
∴an=1+2(n-1)=2n-1.
b1=a2=3,b2=a5=9,
∴等比數列的公比q=3.∴bn
=n;(2)由cb+c
b+…+cnb
n=sn,得cb
+cb+…+c
n?1b
n?1=s
n?1 (n≥2).
兩式作差得:cnb
n=an(n≥2).
∴cn=an?bn(n≥2).又cb
=a,∴c1=a1?b1.
∴cn=an?bn.
∴tn=1×3+3×32+5×33+…+(2n-1)?3n.3tn=1×+3×+…+(2n?1)?n+1
.兩式作差得:?2t
n=3+2(++…+n
)?(2n?1)?n+1
=3+2×9(1?n?1
)1?3
?(2n?1)?n+1.∴t
n=3+(n?1)?n+1.
在公差為d(d≠0)的等差數列{an}和公比為q的等比數列{bn}中,已知a1=b1=1,a2=b2,a8=b3.(1)求數列{a
7樓:小夥
(1)由條件得:
1+d=q
1+7d=q
?d=5
q=6?a
n=5n?4,b
n=n?1
(2)tn=c1+c2+c3+…+cntn=a1b1+a2b2+a3b3+…+an-1bn-1+anbn①qtn=a1b2+a2b3+a3b4+…+an-1bn+anbn+1②
①-②:(1?q)tn=a
b+db
+db+…+db
n?1+dbn?a
nbn+1=a
b+db
(1?q
n?1)
1?q?anb
n+1即 ?5t
n=1+56(1?n?1)?5
?(5n?4)n
∴tn=(n-1)6n+1
已知數列{an}為等差數列,數列{bn}是公比為正的等比數列,且a1=b1=1,a2=b2+1,b4=a4+1 10
8樓:瀧芊
設等差數列公差d,等比數列公比q(q>0)
a2=a1+d=1+d
b2=b1q=q
1+d=q+1, d=q ....(1)
a4=a1+3d=1+3d
b4=b1q^3=q^3
q^3=1+3d+1=2+3d ...(2)
q^3=2+3q, q^3-3q-2=0,q^3+q^2-(q^2+3q+2)=0
q^2(q+1)-(q+1)(q+2)=0, q^2-q-2=0, (q+1)(q-2)=0, q-2=0, q=2
所以 d=2,q=2
an=a1+(n-1)d=1+2(n-1)=2n-1
bn=b1q^(n-1)=2^(n-1)
cn=anbn=(2n-1)*2^(n-1)
tn=1*2^0+3*2^1+5*2^2+....+(2n-3)*2^(n-2)+(2n-1)*2^(n-1) .....(1)
2tn= 1*2^1+3*2^2+...........................+(2n-3)*2^(n-1)+(2n-1)*2^n .....(2)
(1)-(2):
-tn=1*2^0+2*2^1+2*2^2+......+2*2^(n-1)-(2n-1)*2^n
=1+2^2+2^3+....+2^n-(2n-1)*2^n
=1+2*[2^(n-1)-1]/(2-1)-(2n-1)*2^n
=1+2^n-2-(2n-1)*2^n
=-1-(n-1)*2^(n+1)
tn=(n-1)*2^(n+1)+1
t1=(1-1)*2^(1+1)+1=1>=1
假設 tk=(k-1)*2^(k+1)+1>1, k>=1
t(k+1)=(k+1-1)*2^(k+1+1)+1
=2k*2^(k+1)+1
=2k*2^(k+1)-2*2^(k+1)+2*2^(k+1)+1
=2(k-1)*2^(k+1)+2+2^(k+2)-1
=2tk+2^(k+2)-1
>=2*1+2^(1+2)-1=1+8>1
由歸納法知,tn>=1
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