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Algebra Difficulty 4.6 AIME Prove it

Given a sequence {an}\{a_n\} with the first term a1=13a_1= \frac{1}{3} and common ratio q=13q= \frac{1}{3}. Let bn=2log13an1b_n=2\log_{\frac{1}{3}}a_n-1 (nN)(n\in\mathbb{N}^*).

(Ⅰ) Prove that the sequence {bn}\{b_n\} is an arithmetic sequence;
(Ⅱ) Let cn=an+b2nc_n=a_n+b_{2n}, find the sum of the first nn terms of the sequence {cn}\{c_n\}, denoted as TnT_n.

Solution

(Ⅰ) Proof: Since the sequence {an}\{a_n\} has the first term a1=13a_1= \frac{1}{3} and common ratio q=13q= \frac{1}{3},
an=(13)n\therefore a_n=( \frac{1}{3})^{n}, then bn=2log13an1=2log13(13)n1=2n1b_n=2\log_{\frac{1}{3}}a_n-1=2\log_{\frac{1}{3}}( \frac{1}{3})^{n}-1=2n-1.
bn+1bn=[2(n+1)1](2n1)=2\therefore b_{n+1}-b_n=[2(n+1)-1]-(2n-1)=2.
Thus, the sequence {bn}\{b_n\} is an arithmetic sequence with a common difference of 22;
(Ⅱ) Solution: cn=an+b2n=(13)n+(4n1)c_n=a_n+b_{2n}=( \frac{1}{3})^{n}+(4n-1).
\therefore The sum of the first nn terms of the sequence {cn}\{c_n\}, Tn=c1+c2++cn=[13+132++13n]+4(1+2++n)nT_n=c_1+c_2+\ldots+c_n=\left[ \frac{1}{3}+ \frac{1}{3^2}+\ldots+ \frac{1}{3^n}\right]+4(1+2+\ldots+n)-n
=13(113n)113+4(n+1)n2n=12(113n)+2n2+n=2n2+n+12123n= \frac{ \frac{1}{3}(1- \frac{1}{3^n})}{1- \frac{1}{3}}+4\cdot \frac{(n+1)n}{2}-n= \frac{1}{2}(1- \frac{1}{3^n})+2n^2+n=2n^2+n+ \frac{1}{2}- \frac{1}{2\cdot 3^n}.
Thus, the final answer is Tn=2n2+n+12123nT_n = \boxed{2n^2+n+ \frac{1}{2}- \frac{1}{2\cdot 3^n}}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.