Precalculus (6th Edition) Blitzer

Published by Pearson
ISBN 10: 0-13446-914-3
ISBN 13: 978-0-13446-914-0

Chapter 3 - Section 3.4 - Exponential and Logarithmic Equations - Exercise Set - Page 489: 75

Answer

Solution set = $\{12\}$

Work Step by Step

For the equation to be defined, logarithms must have positive arguments $\left[\begin{array}{lllll} x-6 \gt 0, & and & x-4 \gt 0 & and & x \gt 0\\ x \gt 6 & & x \gt 4 & & \end{array}\right]$ $ x \gt 6 \qquad (*)$ - is the condition eventual solutions must satisfy. $\log_{2}(x-6)+\log_{2}(x-4)-\log_{2}x=2$ ... LHS: apply the product rule, ... RHS write $2$ as $\log_{2}2^{2}$ $\log_{6}[(x-6)(x-4)]-\log_{2}x=\log_{2}2^{2}$ ... LHS: apply the quotient rule, $\displaystyle \log_{6}\frac{(x-6)(x-4)}{x}=\log_{2}2^{2}$ ... if $\log_{b}M=\log_{b}N,$ then M=N $\displaystyle \frac{(x-6)(x-4)}{x}=4\quad $ ... multiply with x $ x^{2}-10x+24=4x $ $ x^{2}-14x+24=0$ $(x-12)(x-2)=0$ $ x=2$ - does not satisfy (*), discard $ x=12$ - satisfies (*) Thus, the solution set = $\{12\}$
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