Calculus with Applications (10th Edition)

Published by Pearson
ISBN 10: 0321749006
ISBN 13: 978-0-32174-900-0

Chapter 6 - Applications of the Derivative - 6.5 Related Rates - 6.5 Exercises - Page 341: 8

Answer

$$ y\ln x+xe^{y}=1, \quad [x=1 , \quad y=0 , \quad \frac{dx}{dt}=5] $$ $$ \frac{d y}{d t}=-5 $$

Work Step by Step

$$ y\ln x+xe^{y}=1, \quad [x=1 , \quad y=0 , \quad \frac{dx}{dt}=5] $$ Now, we can calculate $\frac{dy}{dt}$ by implicit differentiation, use the product rule as follows: $$ \begin{aligned} \frac{d}{d t}(y \ln x)+\frac{d}{d x}\left(x e^{y}\right) &=\frac{d}{d t}(1) \\ \ln x \frac{d y}{d t}+\frac{y}{x} \frac{d x}{d t}+e^{y} \frac{d x}{d t}+x \cdot e^{y} \frac{d y}{d t} &=0 \\ \left(\ln x+x e^{y}\right) \frac{d y}{d t}+\left(\frac{y}{x}+e^{y}\right) \frac{d x}{d t} &=0 \\ \left(\ln x+x e^{y}\right) \frac{d y}{d t} &=-\left(\frac{y}{x}+e^{y}\right) \frac{d x}{d t} \\ \frac{d y}{d t} &=-\frac{\left(\frac{y}{x}+e^{y}\right) \frac{d x}{d t}}{\ln x+x e^{y}} \\ &=-\frac{\left(y+x e^{y}\right) \frac{d x}{d t}}{x \ln x+x^{2} e^{y}} \\ & \quad \text { subistituting } \left[\begin{array}{c}{x=1, \\ y=0 ,\\ \frac{dx}{dt}=5 } \end{array}\right]\\ \frac{d y}{d t}&=-\frac{\left[0+(1) e^{0}\right](5)}{(1) \ln 1+1^{2} e^{0}} \\ &=-5 \end{aligned} $$
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