﻿ Proof that the Solution of 3^x = 8 is Irrational

## Proof that the Solution of 3^x = 8 is Irrational

In this post, we discuss another example of proof by contradiction. We are going to prove that the solution of the equation 3^x = 8 is irrational.

Theorem

The solution to the equation $3^x = 8$ is irrational.

Proof

In proof by contradiction, we assume the opposite of the theorem and then find a contradiction somewhere in the proof.

Let us assume the opposite of the theorem. We assume that the solution to the equation $3^x = 8$ is rational.

Now assuming that that the solution to the equation above is rational, then $x$ is a rational number. This means that $x =\frac{p}{q}$, where $p$ and $q$ are integers and $q$ not equal to $0$ (definition of rational numbers). Therefore,

$3^{\frac{p}{q}} = 8$

Raising both sides to $q$, we have $3^p= 8^q$.

Notice that this is a contradiction since $3^p$ is odd and $8^q$ is even and therefore the two numbers cannot be equal. In addition, $3^p = 2^{3q}$ contradicts the Unique Factorization Theorem or the Fundamental Theorem of Arithmetic.

Therefore, our assumption is false and the theorem that $x$ is irrational is true.

21. August 2013 by Guillermo Bautista
Categories: Algebra, Grades 9-12, HS Math | Tags: , , , | 1 comment