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Chapter 11: Problem 2

Write each system of differential equations in matrix form. $$ \begin{array}{l} \frac{d x_{1}}{d t}=x_{1}+x_{2} \\ \frac{d x_{2}}{d t}=-2 x_{2} \end{array} $$

Short Answer

Expert verified
The system is \( \frac{d\mathbf{x}}{dt} = \begin{bmatrix} 1 & 1 \\ 0 & -2 \end{bmatrix} \mathbf{x} \).

Step by step solution

01

Identify the system of equations

The given system of differential equations is: \( \frac{d x_{1}}{d t} = x_{1} + x_{2} \) and \( \frac{d x_{2}}{d t} = -2 x_{2} \).
02

Express the system in a vector form

Let \( \mathbf{x} = \begin{bmatrix} x_{1} \ x_{2} \end{bmatrix} \) and \( \frac{d\mathbf{x}}{dt} = \begin{bmatrix} \frac{dx_{1}}{dt} \ \frac{dx_{2}}{dt} \end{bmatrix} \). The system can be expressed as \( \frac{d\mathbf{x}}{dt} = \begin{bmatrix} x_{1} + x_{2} \ -2x_{2} \end{bmatrix} \).
03

Identify the coefficients of \(x_{1}\) and \(x_{2}\)

From the system, the equation \( \frac{dx_{1}}{dt} = x_{1} + x_{2} \) implies coefficients of 1 for \(x_1\) and 1 for \(x_2\). The equation \( \frac{dx_{2}}{dt} = -2x_{2} \) implies a coefficient of 0 for \(x_1\) and -2 for \(x_2\).
04

Write the coefficient matrix

The coefficients form a matrix \( A = \begin{bmatrix} 1 & 1 \ 0 & -2 \end{bmatrix} \), which represents the relationships in the system.
05

Write the system in matrix form

The system of differential equations can now be expressed in matrix form: \( \frac{d\mathbf{x}}{dt} = A\mathbf{x} \), where \( A = \begin{bmatrix} 1 & 1 \ 0 & -2 \end{bmatrix} \) and \( \mathbf{x} = \begin{bmatrix} x_{1} \ x_{2} \end{bmatrix} \).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

matrix form
Converting a system of differential equations into matrix form is a powerful and systematic approach to simplifying and understanding these equations. By using matrices, we are able to represent multiple equations in a compact, organized manner. This process involves expressing the system of equations in terms of vectors and matrices. For instance, when we have a system of differential equations, each equation can be regarded as a row operation in a matrix. This conversion makes it easier to solve the system using techniques from linear algebra.
  • The matrix form offers a clear visualization of how variables interact with each other via coefficients.
  • This helps in applying efficient computational algorithms to find solutions.
  • Matrix notation is also essential for concepts like eigenvalues and eigenvectors, which are crucial for understanding the stability and behavior of systems.
systems of equations
Systems of equations are a set of equations with multiple variables that we aim to solve simultaneously. These systems often arise in differential equations, where each equation correlates with how a specific variable changes over time. Understanding the relationship among these variables is key to solving the system. By converting these equations into a matrix form, we simplify handling and interpreting complex interactions.
  • Systems of equations can often be represented graphically, demonstrating intersections that correspond to the solutions.
  • In differential equations, focusing on systems highlights how changes in one variable can affect others.
  • Solving these systems can provide valuable insights into real-world phenomena, from physics to economics.
Within differential equations, these systems are modeled to reflect dynamic, process-driven changes, helping to predict future behavior or analyze existing data.
coefficients matrix
The coefficients matrix is a pivotal element when dealing with systems of differential equations. It collects all the coefficients from each equation and organizes them into a matrix. This arrangement allows easy access to the constants that influence the relationships between variables. Consider the matrix as a signature of how the variables interact: the rows represent different equations, while the columns correlate to variables within each equation.
  • The coefficients matrix succinctly encapsulates the structure of the system, making mathematical operations more straightforward.
  • It offers a systematic method to apply algorithms, such as matrix inverses, to solve the system.
  • Determining properties like rank or determinant of this matrix can provide crucial insights into the solvability and nature of the system's solutions.
Finding the coefficients matrix enables efficient manipulation and analysis, whether by hand or with the aid of computer software.

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Most popular questions from this chapter

We consider differential equations of the form $$ \frac{d \mathbf{x}}{d t}=A \mathbf{x}(t) $$ where $$ A=\left[\begin{array}{ll} a_{11} & a_{12} \\ a_{21} & a_{22} \end{array}\right] $$ The eigenvalues of A will be real, distinct, and nonzero. Analyze the stability of the equilibrium \((0,0)\), and classify the equilibrium according to whether it is a sink, a source, or a saddle point.3 $$ A=\left[\begin{array}{rr} -3 & -1 \\ 1 & -6 \end{array}\right] $$

We consider differential equations of the form $$ \frac{d \mathbf{x}}{d t}=A \mathbf{x}(t) $$ where $$ A=\left[\begin{array}{ll} a_{11} & a_{12} \\ a_{21} & a_{22} \end{array}\right] $$ The eigenvalues of \(A\) will be complex conjugates. Analyze the stability of the equilibrium \((0,0)\), and classify the equilibrium according to whether it is a stable spiral, an unstable spiral, or a center. $$ A=\left[\begin{array}{rr} 0 & -1 \\ 1 & 0 \end{array}\right] $$

Analyze the stability of the equilibrium \((0,0)\), and classify the equilibrium. $$ A=\left[\begin{array}{ll} 3 & -5 \\ 2 & -1 \end{array}\right] $$

Solve the given initial-value problem. $$ \left[\begin{array}{c} \frac{d x_{1}}{d t} \\ \frac{d x_{2}}{d t} \end{array}\right]=\left[\begin{array}{ll} 2 & 6 \\ 1 & 3 \end{array}\right]\left[\begin{array}{l} x_{1}(t) \\ x_{2}(t) \end{array}\right] $$ $$ \text { with } x_{1}(0)=-3 \text { and } x_{2}(0)=1 \text { . } $$

Assume that \(N(t)\) denotes prey density at time \(t\) and \(P(t)\) denotes predator density at time \(t\). Their dynamics are given by the system of equations $$ \begin{array}{l} \frac{d N}{d t}=4 N-2 P N \\ \frac{d P}{d t}=P N-3 P \end{array} $$ Assume that initially \(N(0)=3\) and \(P(0)=2\). (a) If you followed this predator-prey community over time, what would you observe? (b) Suppose that bad weather kills \(90 \%\) of the prey population and \(67 \%\) of the predator population. If you continued to observe this predator-prey community, what would you expect to see?
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