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Algebra Difficulty 4.5 AIME Prove it

Given the sequence {an}\{a_n\} such that a1=1a_1=1, a2=3a_2=3, and an+2=3an+12ana_{n+2}=3a_{n+1}-2a_n for nNn\in\mathbb{N}^*.
(1) Prove that the sequence {an+1an}\{a_{n+1}-a_n\} is a geometric sequence;
(2) Let bn=2n1anan+1b_n=\frac{2^{n-1}}{a_n\cdot a_{n+1}} and let TnT_n be the sum of the first nn terms of the sequence {bn}\{b_n\}. Prove that Tn<12T_n<\frac{1}{2}.

Solution

To prove:
(1) Since an+2=3an+12ana_{n+2}=3a_{n+1}-2a_n for nNn\in\mathbb{N}^*,
we have an+2an+1=2(an+1an)a_{n+2}-a_{n+1}=2(a_{n+1}-a_n) for nNn\in\mathbb{N}^*.
Because a2a1=31=2a_2-a_1=3-1=2,
the sequence {an+1an}\{a_{n+1}-a_n\} is a geometric sequence with both initial term and common ratio equal to 2.

(2) From (1) we know that an+1an=2na_{n+1}-a_n=2^n, and clearly, the sequence {an}\{a_n\} is increasing, therefore
bn=2n1anan+1=122nanan+1=12an+1ananan+1=12(1an1an+1),b_n=\frac{2^{n-1}}{a_n\cdot a_{n+1}}=\frac{1}{2}\cdot\frac{2^n}{a_n\cdot a_{n+1}}=\frac{1}{2}\cdot\frac{a_{n+1}-a_n}{a_n\cdot a_{n+1}}=\frac{1}{2}\left(\frac{1}{a_n}-\frac{1}{a_{n+1}}\right),
which leads to
Tn=12(1a11a2+1a21a3++1an1an+1)T_n=\frac{1}{2}\left(\frac{1}{a_1}-\frac{1}{a_2}+\frac{1}{a_2}-\frac{1}{a_3}+\dots+\frac{1}{a_n}-\frac{1}{a_{n+1}}\right)
=12(1a11an+1)=\frac{1}{2}\left(\frac{1}{a_1}-\frac{1}{a_{n+1}}\right)
=12(11an+1)=\frac{1}{2}\left(1-\frac{1}{a_{n+1}}\right)
<12.<\frac{1}{2}.
Hence, we conclude that Tn<12\boxed{T_n<\frac{1}{2}} for all nn.

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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.