Discrete Mathematics with Applications 4th Edition

Published by Cengage Learning
ISBN 10: 0-49539-132-8
ISBN 13: 978-0-49539-132-6

Chapter 4 - Elementary Number Theory and Methods of Proof - Exercise Set 4.7 - Page 213: 24

Answer

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Work Step by Step

**Proof by Contradiction:** Suppose, for the sake of contradiction, that \(\log_5 2\) is rational. Then there exist integers \(p\) and \(q\) with \(q > 0\) such that \[ \log_5 2 = \frac{p}{q}. \] By the definition of logarithms, this equation is equivalent to \[ 5^{\frac{p}{q}} = 2. \] Raising both sides to the power \(q\) yields \[ 5^p = 2^q. \] Now, by the Fundamental Theorem of Arithmetic (unique prime factorization), the prime factors on the left must match those on the right. However, the left side, \(5^p\), is a power of 5 and contains no prime factors other than 5, while the right side, \(2^q\), is a power of 2 and contains no prime factors other than 2. This means that \(5^p\) cannot equal \(2^q\) unless both sides are 1, which is impossible because \(p\) and \(q\) are positive and \(2^q \ge 2\). Thus, we have reached a contradiction. Therefore, \(\log_5 2\) is irrational.
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