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Homework Help: 'Equating the coefficients' question

  1. Sep 24, 2016 #1
    1. The problem statement, all variables and given/known data
    Expand the brackets to get a cubic containing the unknowns. This is an example in the textbook but I don't see how they've expanded the brackets to get their answer:


    2. Relevant equations

    3. The attempt at a solution

    I would have done it how you expand long brackets:

  2. jcsd
  3. Sep 24, 2016 #2


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    Homework Helper

    Yes, and the next step is to collect all the terms involving x^2 together, and all the terms involving x together...
  4. Sep 24, 2016 #3
    What I don't get is how they factorized what you get from expanding: px^3+qx^2+rx-3px^2-3qx-3r. How do you get px^3+(q-3p)x^2+(r-3q)x-3r from that?
  5. Sep 24, 2016 #4
    Ah, I see if you do (q-3p)x^2 and multiply it out and the same with the other factorized terms, you get the terms you get when you'd first expand it out. I hadn't noticed that px^3+(q-3p)x^2... etc. was a cubic. I need to work on noticing things, I've found. :rolleyes:
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