Question #233025
4. Find the sum and product of the eigenvalues of the matrix

2 3 -2
-2 1 1
1 0 2
1
Expert's answer
2021-09-06T13:09:53-0400

Given matrix is:[232211102]\begin{bmatrix} 2 & 3 & -2 \\ -2 & 1 & 1\\ 1 & 0 & 2 \end{bmatrix}

Now, solving 2λ3221λ1102λ=0\begin{vmatrix} 2-\lambda & 3 & -2 \\ -2 & 1-\lambda & 1\\ 1 & 0 & 2-\lambda \end{vmatrix}=0 , we get:

(2λ)[(1λ)(2λ)0]3[2(2λ)1]2[01(1λ)]=0(1λ)(2λ)2+126λ+3+22λ=0(1λ)(2λ)2+178λ=04+λ24λ4λλ3+4λ+178λ=0λ3+λ212λ+21=0λ3λ2+12λ21=0\Rightarrow(2-\lambda)[(1-\lambda)(2-\lambda)-0]-3[-2(2-\lambda)-1]-2[0-1(1-\lambda)]=0\\ \Rightarrow (1-\lambda)(2-\lambda)^2+12-6\lambda+3+2-2\lambda=0\\ \Rightarrow (1-\lambda)(2-\lambda)^2+17-8\lambda=0\\ \Rightarrow 4+\lambda^2-4\lambda-4\lambda-\lambda^3+4\lambda+17-8\lambda=0\\ \Rightarrow -\lambda^3+\lambda^2-12\lambda+21=0\\ \Rightarrow \lambda^3-\lambda^2+12\lambda-21=0\\

So, sum of eigenvalues of matrix is: (11)=1-(\frac{-1}{1})=1

and product of eigenvalues of matrix is: (211)=21-(\frac{21}{1})=-21


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