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Algebra Difficulty 4.6 AIME Prove it

Given a sequence {an}\{a_n\} where a1=1a_1=1, and if 2an+1an=n2n(n+1)(n+2),2a_{n+1}-a_{n}= \frac {n-2}{n(n+1)(n+2)}, bn=an1n(n+1),b_{n}=a_{n}- \frac {1}{n(n+1)},
(1) Prove that {bn}\{b_n\} is a geometric sequence, and find the general formula for {an}\{a_n\};
(2) If Cn=nbnC_n=n b_n, and the sum of the first nn terms is TnT_n, prove that Tn<2T_n<2.

Solution

Proof:
(1) Since 2an+1an=n2n(n+1)(n+2),2a_{n+1}-a_{n}= \frac {n-2}{n(n+1)(n+2)}, bn=an1n(n+1),b_{n}=a_{n}- \frac {1}{n(n+1)},
we have an+1=12an+n22n(n+1)(n+2).a_{n+1}= \frac {1}{2}a_{n}+ \frac {n-2}{2n(n+1)(n+2)}.
Therefore, bn+1bn=12an+n22n(n+1)(n+2)1(n+1)(n+2)an1n(n+1)=12an12×1n(n+1)an1n(n+1)=12.\frac {b_{n+1}}{b_{n}}= \frac { \frac {1}{2}a_{n}+ \frac {n-2}{2n(n+1)(n+2)}- \frac {1}{(n+1)(n+2)}}{a_{n}- \frac {1}{n(n+1)}}= \frac { \frac {1}{2}a_{n}- \frac {1}{2}× \frac {1}{n(n+1)}}{a_{n}- \frac {1}{n(n+1)}}= \frac {1}{2}.
Therefore, {bn}\{b_n\} is a geometric sequence with a common ratio of 12\frac {1}{2}, and the first term is a112=12.a_1- \frac {1}{2}= \frac {1}{2}.
Thus, bn=(12)n.b_n= \left( \frac {1}{2} \right)^{n}.
Hence, an=(12)n+1n(n+1).a_n= \left( \frac {1}{2} \right)^{n}+ \frac {1}{n(n+1)}.

(2) Cn=nbn=n×(12)n.C_n=n b_n=n \times \left( \frac {1}{2} \right)^{n}.
Therefore, Tn=12+2×122+3×123++n×12n,T_n= \frac {1}{2}+2 \times \frac {1}{2^{2}}+3 \times \frac {1}{2^{3}}+\ldots+n \times \frac {1}{2^{n}},
12Tn=122+2×123++(n1)×12n+n×12n+1.\frac {1}{2}T_n= \frac {1}{2^{2}}+2 \times \frac {1}{2^{3}}+\ldots+(n-1) \times \frac {1}{2^{n}}+n \times \frac {1}{2^{n+1}}.
Thus, 12Tn=12+(12)2++(12)nn×12n+1=12(112n)112n×12n+1=12+n2n+1.\frac {1}{2}T_n= \frac {1}{2}+ \left( \frac {1}{2} \right)^{2}+\ldots+\left( \frac {1}{2} \right)^{n}-n \times \frac {1}{2^{n+1}}= \frac { \frac {1}{2}(1- \frac {1}{2^{n}})}{1- \frac {1}{2}}-n \times \frac {1}{2^{n+1}}=1- \frac {2+n}{2^{n+1}}.
Therefore, Tn=22+n2n<2.T_n=2- \frac {2+n}{2^{n}}<2.
Thus, we conclude that Tn<2T_n<2 with Tn<2\boxed{T_n<2}.

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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.