已知等差数列{an}的公差为d(d≠0),等比数列{bn}的公比为q(q>1).设sn=a1b1+a2b2+…+anbn,Tn=a1b1-

2025-06-26 22:15:50
推荐回答(1个)
回答1:

(Ⅰ)解:由题设,可得an=2n-1,bn=3n-1,n∈N*
所以,S3=a1b1+a2b2+a3b3=1×1+3×3+5×9=55
(Ⅱ)证明:由题设可得bn=qn-1则S2n=a1+a2q+a3q2++a2nq2n-1,①
T2n=a1-a2q+a3q2-a4q3+-a2nq2n-1
S2n-T2n=2(a2q+a4q3+-a2nq2n-1
1式加上②式,得S2n+T2n=2(a1+a3q2++a2n-1q2n-2)③
2式两边同乘q,得q(S2n+T2n)=2(a1q+a3q3++a2n-1q2n-1
所以,(1-q)S2n-(1+q)T2n=(S2n-T2n)-q(S2n+T2n
=2d(q+q3++q2n-1
=

2dq(1?q2n)
1?q2
,n∈N*
(Ⅲ)证明:c1-c2=(ak1-al1)b1+(ak2-al2)b2++(akn-aln)bn
=(k1-l1)db1+(k2-l2)db1q++(kn-ln)db1qn-1
因为d≠0,b1≠0,所以
c1?c2
db1
=(k1?l1)+(k2?l2)q++(kn?ln)qn?1

(1)若kn≠ln(2),取i=n
(3)若kn=ln(4),取i满足ki≠li(5)且kj=lj,i+1≤j≤n(6)
由(1),(2)及题设知,1<i≤n
c1?c2
db1
=(k1?l1)+(k2?l2)q+(ki?1?li?1)qi?2+(ki?li)qi?1

1当ki<li2时,得ki-li≤-1,由q≥n,
得ki-li≤q-1,i=1,2,3i-13
即k1-l1≤q-1,(k2-l2)q≤q(q-1),(ki-1-li-1)qi-2≤qi-2(q-1)
又(ki-li)qi-1≤-qi-1
所以
c1?c2
db1
=(q?1)+(q?1)q+(q?1)qi?2?qi?1=(q?1)
1?qi?1
1?q

因此c1-c2≠0,即c1≠c2
4当ki>li5同理可得
c1?c2
db1
<?1
6,因此c1≠c27.综上,c1≠c2