Intermediate Algebra: Connecting Concepts through Application

Published by Brooks Cole
ISBN 10: 0-53449-636-9
ISBN 13: 978-0-53449-636-4

Chapter 1-8 - Cumulative Review - Page 678: 14

Answer

$w=-18$

Work Step by Step

Given \begin{equation} \frac{4}{w^2+2 w-24}=\frac{-0.5 w}{w^2-13 w+36}. \end{equation} Find the zeros of the denominators by factoring. \begin{equation} \begin{aligned} w^2-13 w+36&=0\\ w^2-9 w-4 w+36& =0\\ w(w-9)-4(w-9)&=0 \\ (w-9)(w-4)& =0\\ \implies w= 9 && w= 4 \end{aligned} \end{equation} \begin{equation} \begin{aligned} w^2+2 w-24&=0\\ w^2+6 w-4 w-24& =0\\ w(w+6)-4(w+6)&=0 \\ (w+6)(w-4)& =0\\ \implies w= -6 && w= 4. \end{aligned} \end{equation} Rewrite the original equation and cancel out like terms from both sides. \begin{equation} \begin{aligned} \frac{4}{w^2+2 w-24}&=\frac{-0.5 w}{w^2-13 w+36}\\ \frac{4}{(w+6)(w-4)}&=\frac{-0.5 w}{(w-9)(w-4)}\\ \frac{4}{w+6}&=\frac{-0.5 w}{w-9}. \end{aligned} \end{equation} Cross multiply and solve for $w$: \begin{equation} \begin{aligned} \frac{4}{w+6}&=\frac{-0.5 w}{w-9}\\ 4(w-9)&=-(w+6) \cdot 0.5 w\\ 4(w-9)+(w+6) \cdot 0.5 w&=0\\ 4w-36+0.5w^2+3w&=0\\ 0.5w^2+7w-36&=0\\ w^2+14w-72&= 0\\ w^2+18w-4w-72&= 0\\ w(w+18)-4(w+18)&=0\\ \implies w= -18 \quad w= 4. \end{aligned} \end{equation} Check \begin{equation} \begin{aligned} \frac{4}{(-18)^2+2 \cdot (-18)-24}& \stackrel{?}{=} \frac{-0.5 \cdot (-18)}{(-18)^2-13 \cdot (-18)+36}\\ \frac{1}{66} & =\frac{1}{66}\checkmark\\ \frac{4}{(4)^2+2 \cdot (4)-24}& \stackrel{?}{=} \frac{-0.5 \cdot (4)}{(4)^2-13 \cdot (4)+36}\\ \frac{1}{2} & =\frac{-2}{0}\quad\textbf{False}\\ \end{aligned} \end{equation} Note that $4$ is a zero of the denominators and can not be excepted as an answer. The solution is $w=-18$.
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