Calculus: Early Transcendentals 8th Edition

Published by Cengage Learning
ISBN 10: 1285741552
ISBN 13: 978-1-28574-155-0

Chapter 9 - Section 9.3 - Separable Equations - 9.3 Exercises - Page 605: 15

Answer

$$ x \ln x =y(1+\sqrt {3+y^{2}})y^{\prime }, \quad y(1)=1 . $$ The solution of the given differential equation is $$ \frac{1}{2}x^{2}\ln x-\frac{1}{4} x ^{2}+\frac{41}{12}=\frac{1}{2} y^{2} +\frac{1}{3 }(3+y^{2})^{\frac{3}{2}}. $$

Work Step by Step

$$ x \ln x =y(1+\sqrt {3+y^{2}})y^{\prime }, \quad y(1)=1 . $$ We write the equation in terms of differentials: $$ ( x \ln x) dx =y(1+\sqrt {3+y^{2}})dy $$ and integrate both sides: $$ \int ( x \ln x) dx =\int(y+y\sqrt {3+y^{2}})dy $$ it can be written in the form $$ I_{1} =\frac{1}{2} y^{2} + I_{2} \quad (1) $$ First, we can find the following integral: $$ I_{1}= \int ( x \ln x) dx $$ use integration by parts with $$ \quad\quad\quad \left[\begin{array}{c}{u=\ln x, \quad\quad dv= x dx } \\ {d u= \frac{dx}{x} , \quad\quad v= \frac{1}{2}x^{2} }\end{array}\right] , \text { then }\\ $$ $ \Rightarrow$ $$ I_{1}= \frac{1}{2}x^{2}\ln x-\frac{1}{2}\int x dx=\frac{1}{2}x^{2}\ln x-\frac{1}{4} x ^{2}+C_{1}, \quad (2) $$ where $C_{1} $ is an arbitrary constant. and $$ I_{2}=\int y\sqrt {3+y^{2}}dy $$ put $w=3+y^{2}$ then $dw =2ydy$ so, $$ I_{2}=\frac{1}{2}\int \sqrt {w}dw=\frac{1}{3 }(3+y^{2})^{\frac{3}{2}}+C_{2}, \quad (3) $$ where $C_{2}$ is an arbitrary constant. substituting from Eq. (2)and Eq. (3) in Eq.(1) we have: $$ \frac{1}{2}x^{2}\ln x-\frac{1}{4} x ^{2}+C=\frac{1}{2} y^{2} +\frac{1}{3 }(3+y^{2})^{\frac{3}{2}} $$ where C is an arbitrary constant. Now, to satisfy the condition $ y(1)=1, $we have $$ 0-\frac{1}{4}+C=\frac{1}{2} (1)^{2} +\frac{1}{3 }(3+(1)^{2})^{\frac{3}{2}} $$ $ \Rightarrow$ $$ C=\frac{1}{2} +\frac{8}{3 }+\frac{1}{4}=\frac{41}{12}. $$ So, the solution of the given differential equation is $$ \frac{1}{2}x^{2}\ln x-\frac{1}{4} x ^{2}+\frac{41}{12}=\frac{1}{2} y^{2} +\frac{1}{3 }(3+y^{2})^{\frac{3}{2}} $$
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