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Problem 221

(A) Define the characteristic polynomial of the matrix \(\mathrm{A}\), (B) Let $\mathrm{A}=\begin{array}{ccc}\mid 1 & 2 & -1 \mid \\ \mid 1 & 0 & 1 \mid \\ \mid 4 & -4 & 5\end{array}$ Find the characteristic polynomial of \(\mathrm{A}\).

Short Answer

Expert verified
The characteristic polynomial of the given matrix A is \(p(λ) = -λ^3 + 6λ^2 - 11λ + 6\).
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Step 1: Definition of Characteristic Polynomial

The characteristic polynomial of an n x n matrix A is the polynomial det(A - λI), where λ is the eigenvalue and I is the identity matrix of size n x n. When the polynomial is written in the form of an equation, called the characteristic equation, the determinant of (A - λI) is set to zero and its solutions are the eigenvalues of A. (B) Find the characteristic polynomial of the given matrix A = \(\begin{array}{ccc}\mid 1 & 2 & -1 \mid \\\ \mid 1 & 0 & 1 \\\ \mid 4 & -4 & 5\end{array}\)

Step 1: Calculating (A - λI)

To find the characteristic polynomial, we first need to calculate the matrix (A - λI), where I is the 3x3 identity matrix: \((A - λI) = \begin{bmatrix} 1-λ & 2 & -1 \\ 1 & -λ & 1 \\ 4 & -4 & 5-λ\end{bmatrix}\)

Step 2: Determinant of (A - λI)

Now find the determinant of (A - λI): $$ \operatorname{det}(A - λI) = (1-λ)\begin{vmatrix} -λ & 1 \\ -4 & 5-λ \end{vmatrix} - 2\begin{vmatrix} 1 & 1 \\ 4 & 5-λ \end{vmatrix} - (-1)\begin{vmatrix} 1 & -λ \\ 4 & -4 \end{vmatrix}. $$

Step 3: Expanding the Determinant

Next, expand the determinant using the cofactor expansion: $$ \operatorname{det}(A - λI) = (1-λ)((-λ)(5-λ) - 1(-4)) - 2((1)(5-λ) - 1(4)) + (1(1) - 4(-λ)). $$

Step 4: Simplifying the Expression

Simplify the expression to obtain the characteristic polynomial, p(λ): $$ p(λ) = (1-λ)(λ^2 - 5λ + 4) - 2(-λ + 1) + 1 + 4λ = -λ^3 + 6λ^2 - 11λ + 6. $$ So, the characteristic polynomial of matrix A is \(p(λ) = -λ^3 + 6λ^2 - 11λ + 6\).

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Most popular questions from this chapter

Chapter 9

Find the characteristic polynomials and the eigenvalues of the matrices. (i) \(\begin{array}{rl}\mathrm{A}= & 2 & 3 \\ & 1 & 4\end{array} \mid\) (ii) $\mathrm{B}=\mid \begin{array}{rr}\cos \alpha & \sin \alpha \\ \mid-\sin \alpha & \cos \alpha \mid\end{array}$ (iii) $\begin{array}{rlr}\mathrm{C}= & 1 & 2 & 3 \mid \\ \mid 2 & 1 & 3 \mid \\\ \mid 3 & 3 & 6\end{array} \mid$

Chapter 9

Let \(\mathrm{V}\) be the vector space \(\mathrm{R}^{2}\) and let \(\mathrm{T}\) be the operator defined by $\quad \mathrm{T}(\mathrm{xy})=(2 \mathrm{x}-\mathrm{y}, \mathrm{x}+\mathrm{y})$ Let \(\mathrm{f}(\mathrm{x})=2+3 \mathrm{x}\) and \(\mathrm{g}(\mathrm{x})=\mathrm{x}+\mathrm{x}^{2} .\) Find \(\mathrm{f}(\mathrm{T})\) and \(\mathrm{g}(\mathrm{T})\)

Chapter 9

[A] Find the minimum polynomial \(\mathrm{m}(\lambda)\) of the matrix $\mathrm{A}=\begin{array}{cccc}2 & 1 & 0 & 0 \\ & 0 & 2 & 0 & 0 \mid \\ & 0 & 0 & 2 & 0 \\ & 0 & 0 & 0 & 5\end{array}$ [B] Let \(\mathrm{A}\) be a 3 by 3 matrix over the real field cannot be a zero of the polynomial, \(\varphi(\lambda)=\lambda^{2}+1\)

Chapter 9

Find the minimal polynomials of the following matrices: (i) \(\begin{array}{cc}\mid 3 & 1 \mid \\ \mid 0 & 3 \mid\end{array}\) (ii) $\begin{array}{ccc}\mid 3 & 1 & 0 \\ & \mid 0 & 3 & 0 \mid \\ & \mid 0 & 0 & 3 \mid\end{array}$ (iii) $\begin{array}{ccc}\mid 2 & 0 & 0 \\ & \mid 0 & 3 & 1 \mid \\ \mid 0 & 0 & 3 \mid\end{array}$ (iv) $\begin{array}{rrrr} & 2 & 0 & 0 & 0 \\ & \mid 0 & 2 & 0 & 0 \mid \\ & \mid 0 & 0 & 3 & 0 \\ & 0 & 0 & 0 & 3\end{array}$

Chapter 9

If $\mathrm{F}(\mathrm{X})=\begin{array}{ccccc}1 & 0 & 1 \mid & \mid 2 & 1 & 0 \mid \\ & \mid 2 & 1 & 1 \mid & -\mid-1 & 1 & 1 \\ & 1 & 1 & 1 \mid & \mid 0 & 1 & 0\end{array} \mid$ $\begin{array}{rllll} & \mid 1 & 1 & 1 \mid & & \\ \mathrm{X}+ & \mid 1 & 0 & 1 \mid \mathrm{X}^{2}, & \text { and } \mathrm{B}= & 1 & 1 \\ & \mid 0 & 1 & 0 \mid & 1 & 0 \\ & \text { find } \mathrm{F}_{\mathrm{L}}(\mathrm{B}) & \text { and } \mathrm{F}_{\mathrm{R}}(\mathrm{B}) .\end{array}$

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