已知数列{an}是等差数列,数列{bn}是等比数列,且对任意的n∈N*,都有a1b1+a2b2+a3b3+…+anbn=n?2n+3.

2024-11-11 08:56:24
推荐回答(1个)
回答1:

(1)∵{bn}是等比数列,首项为4,公比为2,
∴bn=4?2n-1=2n+1
∵数列{an}是等差数列,且对任意的n∈N*
都有a1b1+a2b2+a3b3+…+anbn=n?2n+3
∴a1b1=24,∴a1

24
b1
24
4
=4,
a1b1+a2b2=2?25
a2b2=2?25?24=48
∴a2=
48
b2
=
48
23
=6,
∴d=a2-a1=6-4=2,
∴an=4+(n-1)×2=2n+2.
∴Sn=(a1+a2+a3+…+an)+(b1+b2+…+bn
=[4n+
n(n?1)
2
×2
]+
4(1?2n)
1?2

=n2+3n+2n+2-4.
(2)①∵a1=8,a1b1+a2b2+a3b3+…+anbn=n?2n+3
∴8b1=24,解得b1=2,
设等差数列{an}的公差为d,等比数列{bn}的公比为q,
16+(8+d)?2q=2?25
2?25+(8+2d)?2q2=3?26

解得d=4,q=2