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: 100

Answer

The solution set is $\displaystyle \{\frac{-5+\sqrt{37}}{2}\}.$

Work Step by Step

First, in order for the equation to be defined, $\left[\begin{array}{lll} x+5 \gt 0 & \Rightarrow & x \gt -5\\ x\gt 0 & & \end{array}\right] \Rightarrow x \gt 0 \quad (*)$ $\ln 3-\ln(x+5)-\ln x=0$ Apply the quotient rule, ... $0=\ln 1$ $\displaystyle \ln\frac{3}{(x+5)}-\ln x=\ln 1$ Apply the quotient rule, $\displaystyle \ln\frac{3}{x(x+5)}=\ln 1$ Logarithmic functions are one-to-one $\displaystyle \frac{3}{x(x+5)}=1$ $3=x(x+5)$ $ x^{2}+5x=3$ $ x^{2}+5x-3=0$ Use the quadratic formula $ x=\displaystyle \frac{-b\pm\sqrt{b^{2}-4ac}}{2a}$ $ x=\displaystyle \frac{-(5)\pm\sqrt{(5)^{2}-4(1)(-3)}}{2(1)}=\frac{-5\pm\sqrt{37}}{2}$ $ x=\displaystyle \frac{-5+\sqrt{37}}{2}\approx 0.54\quad $ satisfies (*) $ x=\displaystyle \frac{-5-\sqrt{37}}{2}\approx-5.54 \quad $ does not satisfy (*), discard. The solution set is $\displaystyle \{\frac{-5+\sqrt{37}}{2}\}.$
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