21. Let Ax = 0 be a homogeneous system of n linear equations in n unknowns that has only the trivial solution. Prove that if k is any positive integer, then the system Akx = 0 also has only the trivial solution.

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Chapter2: Second-order Linear Odes
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Ff1.6 question 21 on paper please
### Solving Matrix Equations

#### In Exercises 19–20, solve the matrix equation for \( X \).

19. \[
\begin{pmatrix}
1 & -1 & 1 \\
2 & 3 & 0 \\
0 & 2 & -1 
\end{pmatrix}
X = 
\begin{pmatrix}
2 & -1 & 5 & 7 & 8 \\
4 & 0 & -3 & 0 & 1 \\
3 & 5 & -7 & 2 & 1
\end{pmatrix}
\]

20. \[
\begin{pmatrix}
-2 & 0 & 1 \\
0 & -1 & -1 \\
1 & 1 & -4
\end{pmatrix}
X = 
\begin{pmatrix}
4 & 3 & 2 & 1 \\
6 & 7 & 8 & 9 \\
1 & 3 & 7 & 9
\end{pmatrix}
\]

#### Working with Proofs

21. Let \( A\mathbf{x} = 0 \) be a homogeneous system of \( n \) linear equations in \( n \) unknowns that has only the trivial solution. Prove that if \( k \) is any positive integer, then the system \( A^{k}\mathbf{x} = 0 \) also has only the trivial solution.

22. Let \( A\mathbf{x} = 0 \) be a homogeneous system of \( n \) linear equations in \( n \) unknowns, and let \( Q \) be an invertible \( n \times n \) matrix. Prove that \( A\mathbf{x} = 0 \) has only the trivial solution if and only if \( (QA)\mathbf{x} = 0 \) has only the trivial solution.

23. Let \( A\mathbf{x} = \mathbf{b} \) be any consistent system of linear equations, and let \( \mathbf{x_1} \) be a fixed solution. Prove that every solution to the system can be written as \( \mathbf{x} = \mathbf{x_1} + \mathbf{x_2} \), where \( \mathbf{x_2} \) is a solution to the corresponding homogeneous system \( A\mathbf{x} = 0 \).

---
Transcribed Image Text:### Solving Matrix Equations #### In Exercises 19–20, solve the matrix equation for \( X \). 19. \[ \begin{pmatrix} 1 & -1 & 1 \\ 2 & 3 & 0 \\ 0 & 2 & -1 \end{pmatrix} X = \begin{pmatrix} 2 & -1 & 5 & 7 & 8 \\ 4 & 0 & -3 & 0 & 1 \\ 3 & 5 & -7 & 2 & 1 \end{pmatrix} \] 20. \[ \begin{pmatrix} -2 & 0 & 1 \\ 0 & -1 & -1 \\ 1 & 1 & -4 \end{pmatrix} X = \begin{pmatrix} 4 & 3 & 2 & 1 \\ 6 & 7 & 8 & 9 \\ 1 & 3 & 7 & 9 \end{pmatrix} \] #### Working with Proofs 21. Let \( A\mathbf{x} = 0 \) be a homogeneous system of \( n \) linear equations in \( n \) unknowns that has only the trivial solution. Prove that if \( k \) is any positive integer, then the system \( A^{k}\mathbf{x} = 0 \) also has only the trivial solution. 22. Let \( A\mathbf{x} = 0 \) be a homogeneous system of \( n \) linear equations in \( n \) unknowns, and let \( Q \) be an invertible \( n \times n \) matrix. Prove that \( A\mathbf{x} = 0 \) has only the trivial solution if and only if \( (QA)\mathbf{x} = 0 \) has only the trivial solution. 23. Let \( A\mathbf{x} = \mathbf{b} \) be any consistent system of linear equations, and let \( \mathbf{x_1} \) be a fixed solution. Prove that every solution to the system can be written as \( \mathbf{x} = \mathbf{x_1} + \mathbf{x_2} \), where \( \mathbf{x_2} \) is a solution to the corresponding homogeneous system \( A\mathbf{x} = 0 \). ---
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