Given a sequence {an} with the first term a1=31 and common ratio q=31. Let bn=2log31an−1(n∈N∗).
(Ⅰ) Prove that the sequence {bn} is an arithmetic sequence; (Ⅱ) Let cn=an+b2n, find the sum of the first n terms of the sequence {cn}, denoted as Tn.
Solution
(Ⅰ) Proof: Since the sequence {an} has the first term a1=31 and common ratio q=31, ∴an=(31)n, then bn=2log31an−1=2log31(31)n−1=2n−1. ∴bn+1−bn=[2(n+1)−1]−(2n−1)=2. Thus, the sequence {bn} is an arithmetic sequence with a common difference of 2; (Ⅱ) Solution: cn=an+b2n=(31)n+(4n−1). ∴ The sum of the first n terms of the sequence {cn}, Tn=c1+c2+…+cn=[31+321+…+3n1]+4(1+2+…+n)−n =1−3131(1−3n1)+4⋅2(n+1)n−n=21(1−3n1)+2n2+n=2n2+n+21−2⋅3n1. Thus, the final answer is Tn=2n2+n+21−2⋅3n1.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: NuminaMath-1.5,
licensed Apache-2.0.
Statement and solution reproduced as published; topic and difficulty added by this site.