Question

Find the characteristic polynomial of each of the following matrices:

(a)[2113](b)[121012132] (c)[413021003](d)[4233]\begin{align*} & (a)\begin{bmatrix} 2 & 1\\ -1 & 3 \end{bmatrix} & & (b) \begin{bmatrix} 1 & 2 & 1\\ 0 & 1 & 2\\ -1 & 3 & 2 \end{bmatrix} \\\ \\ & (c) \begin{bmatrix} 4 & -1 & 3\\ 0 & 2 & 1\\ 0 & 0 & 3 \end{bmatrix} & & (d) \begin{bmatrix} 4 & 2 \\ 3 & 3 \end{bmatrix} \end{align*}

Solution

Verified
Step 1
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(a) Let

A=[2113]A=\begin{bmatrix} \phantom{-}2 & \phantom{-}1\\ -1 & \phantom{-}3 \end{bmatrix}

Then

(xI2A)=[x211x3](xI_2-A)=\begin{bmatrix} \phantom{-}x-2 & -1\\ \phantom{-}1 & \phantom{-}x-3 \end{bmatrix}

Let characteristic polynomial of AA be p(x)p(x). Then

p(x)= det(xI2A)=(x2)(x3)+1=x25x+7p(x)=\text{ det}(xI_2-A)=(x-2)(x-3)+1=x^2-5x+7

Thus the required characteristic polynomial is x25x+7x^2-5x+7.

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