Algebra and Trigonometry 10th Edition

Published by Cengage Learning
ISBN 10: 9781337271172
ISBN 13: 978-1-33727-117-2

Chapter 5 - 5.4 - Exponential and Logarithmic Equations - 5.4 Exercises - Page 396: 70

Answer

Graphically: $x=2.264$ Algebraically: $x=2.264$

Work Step by Step

Let $y_1=\ln (x+1)$ and $y_2=2-\ln (x)$. The graph is as shown using a graphing utility. From the graph, the intersection point is at $(2.264,1.183)$. Thus, graphically, the solution is $x=2.264$. Solving algebraically: $$\ln (x+1)+\ln (x)=2-\ln (x)+\ln (x)$$ $$\ln (x+1)x=2$$ $$\ln (x^2+x)=2$$ Applying log rules which is $\ln(x)=y$ is equivalent to $x=e^y$: $$x^2+x=e^2$$ $$x^2+x-e^2=0$$ $$x=\frac{-1\pm\sqrt{1^2-4(1)(-e^2)}}{2(1)}=\frac{-1\pm\sqrt{1+4e^2}}{2}$$ $$x_1=\frac{-1+\sqrt{1+4e^2}}{2}$$ $$x_2=\frac{-1-\sqrt{1+4e^2}}{2}$$ Checking $x_1$: $$\ln \left(\frac{-1+\sqrt{1+4e^2}}{2}+1\right)=2-\ln \frac{-1+\sqrt{1+4e^2}}{2}$$ $$1.183=1.183~True$$ Checking $x_2$: $$\ln \left(\frac{-1-\sqrt{1+4e^2}}{2}+1\right)=2-\ln \frac{-1-\sqrt{1+4e^2}}{2}$$ $$undefined=undefined\Rightarrow Not~valid$$ Simplifying $x_1$: $$x_1=\frac{-1+\sqrt{1+4e^2}}{2}=2.264$$ Thus, algebraically, the solution is $x=\frac{-1+\sqrt{1+4e^2}}{2}=2.264$.
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