Part of ALG-11 — Logarithms, Exponentials & Functional Equations

Logarithmic Inequalities

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The base determines inequality direction. For base a > 1: logalog_a(f(x)) > logalog_a(g(x)) iff f(x) > g(x), with both f(x), g(x) > 0. For base 0 < a < 1: the inequality REVERSES — logalog_a(f(x)) > logalog_a(g(x)) iff f(x) < g(x), with both positive. When the base is variable, split into two cases. For logalog_a(f(x)) > k: if a > 1, then f(x) > aka^k (and f(x) > 0); if 0 < a < 1, then 0 < f(x) < aka^k. The most common JEE pattern: solve log1/2log_{1/2}(expression) > 0, which means 0 < expression < 1 (since base < 1 and log1/2log_{1/2}(1) = 0). Always intersect the algebraic solution with the domain condition.

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