Maths Olympiad Prep

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Algebra Difficulty 5.0 AIME, harder Prove it Philippines

Problem:

Arrange these four numbers from smallest to largest: log32\log_{3} 2, log53\log_{5} 3, log62575\log_{625} 75, 23\frac{2}{3}.

Solution

Solution:

The numbers, arranged from smallest to largest, are log32\log_{3} 2, 23\frac{2}{3}, log62575\log_{625} 75, and log53\log_{5} 3.

- Since (3log32)3=8\left(3^{\log_{3} 2}\right)^{3} = 8 and (323)3=9\left(3^{\frac{2}{3}}\right)^{3} = 9, then log32<23\log_{3} 2 < \frac{2}{3}.

- Since (62523)3=58=5625\left(625^{\frac{2}{3}}\right)^{3} = 5^{8} = 5^{6} \cdot 25 and (625log62575)3=753=5627\left(625^{\log_{625} 75}\right)^{3} = 75^{3} = 5^{6} \cdot 27, then 23<log62575\frac{2}{3} < \log_{625} 75.

- If A=log62575A = \log_{625} 75, then 54A=755^{4A} = 75. On the other hand, 54log53=815^{4 \log_{5} 3} = 81. Thus, log62575<log53\log_{625} 75 < \log_{5} 3.

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