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Algebra Difficulty 4.7 AIME Prove it Singapore

Let a1,a2,,a9a_1, a_2, \dots, a_9 be a sequence of numbers satisfying 0<paiq0 < p \le a_i \le q for each i=1,2,,9i = 1, 2, \dots, 9. Prove that
a1a9+a2a8++a9a11+4(p2+q2)pq. \frac{a_1}{a_9} + \frac{a_2}{a_8} + \dots + \frac{a_9}{a_1} \le 1 + \frac{4(p^2 + q^2)}{pq}.

Solution

Note that for any positive numbers a,ba, b between pp and qq, we may assume (due to symmetry) that 0<pabq0 < p \le a \le b \le q. We have
ab+bapq+qp    a2+b2abp2+q2pq    p2ab+q2aba2pqb2pq=(bqap)(aqbp)0 \frac{a}{b} + \frac{b}{a} \le \frac{p}{q} + \frac{q}{p} \iff \frac{a^2 + b^2}{ab} \le \frac{p^2 + q^2}{pq} \\ \iff p^2 ab + q^2 ab - a^2 pq - b^2 pq = (bq - ap)(aq - bp) \ge 0
Since the last inequality is true, the first is true as well. Thus
LHS=(a1a9+a9a1)++(a4a6+a6a4)+a5a51+4(p2+q2)pq \text{LHS} = \left(\frac{a_1}{a_9} + \frac{a_9}{a_1}\right) + \dots + \left(\frac{a_4}{a_6} + \frac{a_6}{a_4}\right) + \frac{a_5}{a_5} \le 1 + \frac{4(p^2 + q^2)}{pq}

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.