Differential Equations and Linear Algebra (4th Edition)

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

Chapter 9 - Systems of Differential Equations - 9.3 General Results for First-Order Linear Differential Systems - Problems - Page 598: 7

Answer

See below

Work Step by Step

Given: $x_1(t)=\begin{bmatrix} t\sin t\\ \cos t \end{bmatrix}$ $x_2(t)=\begin{bmatrix} -t\cos t\\ \sin t \end{bmatrix}$ $A=\begin{bmatrix} \frac{1}{t} & t\\ -\frac{1}{t} & 0 \end{bmatrix}$ Differentiating the given vector functions with respect to t yields, $x_1'(t)=\begin{bmatrix} t\cos t+\sin t\\ -\sin t \end{bmatrix}$ $x_2'(t)=\begin{bmatrix} t\sin t-\cos t\\ \cos t \end{bmatrix}$ whereas $Ax_1(t)=\begin{bmatrix} \frac{1}{t} & t\\ -\frac{1}{t} & 0 \end{bmatrix}\begin{bmatrix} t\sin t\\ \cos t \end{bmatrix}=\begin{bmatrix} t\cos t+\sin t\\ -\sin t \end{bmatrix}=x_1'(t)$ $Ax_2(t)=\begin{bmatrix} \frac{1}{t} & t\\ -\frac{1}{t} & 0 \end{bmatrix}\begin{bmatrix} -t\cos t\\ \sin t \end{bmatrix}=\begin{bmatrix} t\sin t-\cos t\\ \cos t \end{bmatrix}=x_2'(t)$ The Wronskian of these solutions is $W_{[x_1,x_2]}(t)=\begin{vmatrix} t\sin t & -t\cos t\\ \cos t &\sin t \end{vmatrix}=t$ Since the Wronskian is never zero, it follows that $\{x_1, x_2\}$ is linearly independent on any interval and so is a fundamental set of solutions for the given vector differential equation. Therefore, the general solution to the system is $x(t)=\begin{bmatrix} c_1t\sin t-c_2t\cos t \\ c_1\cos t+c_2\sin t \end{bmatrix}$
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