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Homework Help: Eigenvalues and Eigenvectors

  1. May 13, 2016 #1
    1. The problem statement, all variables and given/known data
    we have this matrix
    6 - 1 0
    -1 -1 -1
    0 -1 1
    We need to find it's eigenvalues and eigenvectors

    2. Relevant equations 3. The attempt at a solution

    I wrote the characteristic equation - det(A- λxunit matrix) to find the roots and got (-λ^3)+8(λ^2)+λ-6 instead of -λ(^3)+6(λ^2)+3λ-13, which restricts me from getting the eigenvalues and vectors in the end. I don't think I'm expanding the determinant correctly, even though I know the -1 on r1, c2 turns into a +.
    Do I have to apply cofactors to every row, or just to the coefficients of the 2x2 matrix determinants (6 -(-1) and 0)

    These are the supposed answers
  2. jcsd
  3. May 13, 2016 #2

    Buzz Bloom

    User Avatar
    Gold Member

    Hi kev:

    You need to calculate the determinant as the sum of six products, each with an appropriate +/- sign. Each product includes one element from each row and each column.

    See https://en.wikipedia.org/wiki/Determinant .

    Also, you may have forgotten that the cells along the main diagonal all have a "-λ" added to the numerical value in the cell.

    Hope this helps.

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