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Spring Mathematical Tournament

Algebra Difficulty 4.8 AIME Prove it Bulgaria

Find all values of the real parameter aa such that the inequality
log⁡a(ax+1)+1log⁡a2−1a≤x−1+log⁡a(a2−1) \log_a(a^x + 1) + \frac{1}{\log_{a^2-1} a} \le x - 1 + \log_a(a^2 - 1)
holds true for every x∈(0,1]x \in (0, 1].

Solution

We have that a>1a > 1, x>0x > 0 and x≠log⁡a2x \neq \log_a 2. Then the inequality is equivalent to
(ax−1−1)(ax+1+1)≤0, (a^{x-1} - 1)(a^{x+1} + 1) \le 0,
giving x≤1x \le 1. Since log⁡a2>0\log_a 2 > 0 and log⁡a2≤1\log_a 2 \le 1 when a≥2a \ge 2 we obtain:

1. For 1<a<21 < a < 2 the inequality holds true iff 0<x≤10 < x \le 1.

2. For a≥2a \ge 2 the inequality holds true iff 0<x≤10 < x \le 1 and x≠log⁡a2<1x \neq \log_a 2 < 1.

Therefore the desired values are a∈(1,2)a \in (1, 2).

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