Thursday, 18 May 2017

proof verification - Prove $log_{4}6$ is irrational

Thanks for taking the time to verify my approach and as well as my answer.



Background:





  • B.S. in Business from a 4-year university taking CS courses online

  • I would like some help with a basic proof from MIT's 6.042J Mathematics for Computer Science course.



The question is:
Prove $\log_{4}6$ is irrational.



We prove the contradiction.





  • Suppose $\log_{4}6$ is rational (i.e. a quotient of integers)
    $$\log_{4}6 = m/n$$

  • So we must have m, n integers without common prime factors such that
    $$4^{m/n} = 6$$

  • We will show that m and n are both even
    $$(4^{m/n})^{n} = 6^{n}$$

  • So
    $$4^{m} = 6^{n}$$


  • We then divide the two base numbers by their common factor, $2$, which gives us:



$$2^{m} = 3^{n}$$




  • Since the product of two even numbers must be even AND the product of two odd numbers must be odd, $2^{m}$ and $3^{n}$ are not equivalent and therefore $m/n$ must not be rational.



Q.E.D. We conclude that $\log_{4}6$ is irrational.

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