Differential Equations and Linear Algebra (4th Edition)

Published by Pearson
ISBN 10: 0-32196-467-5
ISBN 13: 978-0-32196-467-0

Chapter 1 - First-Order Differential Equations - 1.12 Chapter Review - Additional Problems - Page 109: 10

Answer

See below

Work Step by Step

Given: $y'=y(y-2)(y-1)$ Rewrite as: $\frac{dy}{dx}=y(y-2)(y-1)\\\rightarrow \frac{dy}{y(y-2)(y-1)}=dx$ Integrate both sides: $\int \frac{dy}{y(y-2)(y-1)}=\int dx$ To solve this: $\frac{dy}{y(y-2)(y-1)}=\frac{A}{y}+\frac{A}{y-2}+\frac{C}{y-1}=\frac{(A+B+C)y^2+(-3A-B-2C)y+2A}{y(y-2)(y-1)}$ We have the system: $A+B+C=0\\3A+B+2C=0\\2A=1$ then $A=\frac{1}{2}\\ B+C=-\frac{1}{2}\\ B+2C=-\frac{3}{2}\\ \rightarrow C=-1\\ \rightarrow B=\frac{1}{2}$ Hence, $\int \frac{dy}{y(y-2)(y-1)}=\int\frac{dy}{2y}+\int\frac{dy}{2(y-2)}-\int\frac{dy}{y-1}\\ =\frac{1}{2}\int\frac{dy}{y}+\frac{1}{2}\int \frac{dy}{y-2}-\int\frac{dy}{y-1}\\ =\frac{1}{2}\ln(y)+\frac{1}{2}\ln (y-2)-\ln(y-1)\\ =\frac{1}{2}(\ln(y)+\ln(y-2))-\ln(y-1))\\ =\frac{1}{2}\ln(y(y-2))-\ln(y-1)$ Solve this, we get $\frac{1}{2}\ln(y(y-2))-\ln(y-1)=x+c$ where $c_1=0\\c_2=2\\c_3=4\\c_4=8$
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