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Algebra Difficulty 6.2 National olympiad Prove it

32. Let the positive integer n2,a1,a2,,ann \geqslant 2, a_{1}, a_{2}, \cdots, a_{n} be nn non-negative real numbers, prove the inequality: (a13+\left(a_{1}^{3}+\right. 1) (a23+1)(an3+1)(a12a2+1)(a22a3+1)(an2a1+1)\left(a_{2}^{3}+1\right) \cdots\left(a_{n}^{3}+1\right) \geqslant\left(a_{1}^{2} a_{2}+1\right)\left(a_{2}^{2} a_{3}+1\right) \cdots\left(a_{n}^{2} a_{1}+1\right). (2001 Czech-Slovak-Polish Joint Competition Problem)

Solution

32. From the generalization of Cauchy's inequality, we have
(ak3+1)(ak3+1)(ak+13+1)(ak2ak+1+1)3,k=1,2,,n, \left(a_{k}^{3}+1\right)\left(a_{k}^{3}+1\right)\left(a_{k+1}^{3}+1\right) \geqslant\left(a_{k}^{2} a_{k+1}+1\right)^{3}, \quad k=1,2, \cdots, n,
where an+1=a1a_{n+1}=a_{1}.

Multiplying them together, we get
k=1n(ak3+1)3k=1n(ak2ak+1+1)3 \prod_{k=1}^{n}\left(a_{k}^{3}+1\right)^{3} \geqslant \prod_{k=1}^{n}\left(a_{k}^{2} a_{k+1}+1\right)^{3}

which simplifies to
(a13+1)(a23+1)(an3+1)(a12a2+1)(a22a3+1)(an2a1+1) \left(a_{1}^{3}+1\right)\left(a_{2}^{3}+1\right) \cdots\left(a_{n}^{3}+1\right) \geqslant\left(a_{1}^{2} a_{2}+1\right)\left(a_{2}^{2} a_{3}+1\right) \cdots\left(a_{n}^{2} a_{1}+1\right)

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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.