已知等差數列an的公差d 0,數列bn是等比數列又a

時間 2021-05-05 23:18:53

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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