Question

Write the given linear system in matrix form;

$$\begin{gathered} \frac{\mathbf{d}\mathbf{x}}{\mathbf{d}\mathbf{t}}\mathbf{=}\mathbf{-}\mathbf{3}\mathit{x}\mathbf{+}\mathbf{4}\mathit{y}\mathbf{+}\mathbf{-}\mathbf{9}\mathit{z} \\ \frac{\mathbf{d}\mathbf{y}}{\mathbf{d}\mathbf{t}}\mathbf{=}\mathbf{6}\mathit{x}\mathbf{-}\mathit{y} \\ \frac{\mathbf{d}\mathbf{z}}{\mathbf{d}\mathbf{t}}\mathbf{=}\mathbf{10}\mathit{x}\mathbf{+}\mathbf{4}\mathit{y}\mathbf{+}\mathbf{3}\mathit{z} \end{gathered}$$

Short answer

Matrix form of the given linear system is$X\text{'}=\left(\begin{array}{ccc}-3& 4& -9\\ 6& -1& 0\\ 10& 4& 3\end{array}\right)X$

Step-by-step solution

Matrix form of a Linear System

If X,A(t),and F(t) denote the respective matrices

$\mathrm{X}=\left(\begin{array}{l}{\mathrm{x}}_1\left(\mathrm{t}\right)\\ {\mathrm{x}}_2\left(\mathrm{t}\right)\\ .\\ .\\ .\\ {\mathrm{x}}_{\mathrm{n}}\left(\mathrm{t}\right)\end{array}\right),\mathrm{A}\left(\mathrm{t}\right)=\left(\begin{array}{ccc}{\mathrm{a}}_{11}\left(\mathrm{t}\right)& \dots & {\mathrm{a}}_{1\mathrm{n}}\left(\mathrm{t}\right)\\ ⋮& :& ⋮\\ {\mathrm{a}}_{\mathrm{m}1}\left(\mathrm{t}\right)& \cdots & {\mathrm{a}}_{\mathrm{mn}}\left(\mathrm{t}\right)\end{array}\right),\mathrm{F}\left(\mathrm{t}\right)=\left(\begin{array}{l}{\mathrm{f}}_1\left(\mathrm{t}\right)\\ {\mathrm{f}}_2\left(\mathrm{t}\right)\\ .\\ .\\ .\\ {\mathrm{f}}_{\mathrm{n}}\left(\mathrm{t}\right)\end{array}\right)$

Then the system of linear first order differential equations (3) can be written as,

$\frac{\mathrm{d}}{\mathrm{dt}}\left(\begin{array}{l}{\mathrm{x}}_1\left(\mathrm{t}\right)\\ {\mathrm{x}}_2\left(\mathrm{t}\right)\\ .\\ .\\ .\\ {\mathrm{x}}_{\mathrm{n}}\left(\mathrm{t}\right)\end{array}\right)=\left(\begin{array}{ccc}{\mathrm{a}}_{11}\left(\mathrm{t}\right)& \dots & {\mathrm{a}}_{1\mathrm{n}}\left(\mathrm{t}\right)\\ ⋮& :& ⋮\\ {\mathrm{a}}_{\mathrm{m}1}\left(\mathrm{t}\right)& \cdots & {\mathrm{a}}_{\mathrm{mn}}\left(\mathrm{t}\right)\end{array}\right)\left(\begin{array}{l}{\mathrm{x}}_1\\ {\mathrm{x}}_2\\ .\\ .\\ .\\ {\mathrm{x}}_{\mathrm{n}}\end{array}\right)+\left(\begin{array}{l}{\mathrm{f}}_1\left(\mathrm{t}\right)\\ {\mathrm{f}}_2\left(\mathrm{t}\right)\\ .\\ .\\ .\\ {\mathrm{f}}_{\mathrm{n}}\left(\mathrm{t}\right)\end{array}\right)$

Or simply X’=AX+F

If the system is homogenous, its matrix form is then

X’=AX

Changing the linear system in matrix form:

In the following question we will change the linear form in its matrix form;

$$\begin{gathered} \left\{\frac{dx}{dt}=-3x+4y+-9z \\ \frac{dy}{dt}=6x-y \\ \frac{dz}{dt}=10x+4y+3z\right. \end{gathered}$$

$$\begin{gathered} X\text{'}=\left(\begin{array}{ccc}-3& 4& -9\\ 6& -1& 0\\ 10& 4& 3\end{array}\right)X \\ A=\left(\begin{array}{ccc}-3& 4& -9\\ 6& -1& 0\\ 10& 4& 3\end{array}\right) \\ X=\left(\begin{array}{c}x\\ y\\ z\end{array}\right) \end{gathered}$$

Hence, the final answer is$X\text{'}=\left(\begin{array}{ccc}-3& 4& -9\\ 6& -1& 0\\ 10& 4& 3\end{array}\right)X$