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Simple Factoring Problem

  1. Feb 20, 2007 #1
    1. The problem statement, all variables and given/known data

    [tex]28s^2 + 8st - 20t^2[/tex]

    The answer in the back of the book is 4(7s - 5t)(s + t)

    2. The attempt at a solution

    [tex]4(7s^2 + st + st - 5t^2)[/tex]
    [tex]4(7s^2 + st + st - 20t^2)[/tex]
    [tex]4(s(7s + t) + t(s - 20t))[/tex]

    I'm lost, can someone please help me?

    Thanks in advance.
    Last edited: Feb 20, 2007
  2. jcsd
  3. Feb 20, 2007 #2
    In your first attempt at the solution, why not combine the second and third terms inside the (), it might make things more obvious.
  4. Feb 20, 2007 #3
    Is there some trick to this that I am missing?

    [tex]4(7s^2 + 2st - 5t^2)[/tex]
  5. Feb 20, 2007 #4


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    Well, you could note that 2st=7st-5st, which makes your expression 4(7s2+7st-5st-5t2). Is this any easier to factorise?
  6. Feb 20, 2007 #5

    Oh my god, how could I be so blind? That t^2 at the end threw me off. I feel like such an idiot. Thanks a billion cristo and robb, and I apologize for wasting your time :)
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