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Homework Help: Why can this be false?

  1. Oct 31, 2005 #1
    (e+f)^2 = e^2+2ef+f^2
    Why is this not neccesary true for squares matrices for all size?
     
  2. jcsd
  3. Nov 1, 2005 #2

    HallsofIvy

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    (e+ f)^2= (e+f)(e+f)= e^2+ fe+ef+ fe. Remember that multiplication of matrices is not, in general, commutative!
     
  4. Nov 8, 2005 #3
    Do you mean (e+f)2 = e2+fe+ef+f2? I might be misinterpreting it but I can't see where that extra "fe" came from and where the "f2" went.
     
  5. Nov 8, 2005 #4

    AKG

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    Yeah, that's what he meant: (e+f)² = e² + ef + fe + f².
     
  6. Nov 9, 2005 #5

    HallsofIvy

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    One of these days, I'm going to learn to type!
     
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