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Homework Help: Idempotent matrices

  1. Sep 15, 2009 #1
    Guys, I really need your help. I've been working on this problem all day, and I'm starting to pull my hair out.

    I have three idempotent 3x3 matrices X1, X2 and X3.

    Let A=aX1+bX2+cX3 where a, b, c are scalars

    Show that AXi=Xi (where i=1, 2, 3)

    I get A=[a,a,a;b,b,b;c,c,c], but then I have AXi=A... What am I doing wrong?
  2. jcsd
  3. Sep 15, 2009 #2


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    welcome to pf!

    hi sam! welcome to pf! :smile:
    yes, i get the same :smile:

    it must be a misprint for "Show that AXi = A" :rolleyes:
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