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Homework Help: Multiplying a 3 by 3 matrix

  1. Feb 26, 2012 #1
    1. The problem statement, all variables and given/known data

    In the following problem they are taking the third derivative, then multiplying a 3 by 3 matrix. I don't understand how they progress from the first to the second to the third deriviative in the following two questions.

    These are the parts of the solution that I really don't understand


    What's going on? -3 sin t is not the derivative of -3 sin t


    The derivative of e^t(cos t - sin t) is (e^t)(-sin t - cos t) not 2e^t sin t
    The derivative of e^t(sin t + cos t) is (e^t)(cos t - sin t) not (2e^t)(cos t)

    I don't understand.
    Last edited: Feb 26, 2012
  2. jcsd
  3. Feb 26, 2012 #2
    Re: torsion

    I looked at some more examples and they take the derivative for all the others and I understand them. I don't however understand the derivatives of the two above.
  4. Feb 27, 2012 #3


    Staff: Mentor

    Re: torsion

    Looks like a typo to me. The middle row in the 2nd column should be -3cos(t).
    Your work here is incorrect. Both functions are products, so you need to use the product rule when you differentiate.
  5. Feb 27, 2012 #4
    Re: torsion

    excellent. i got it now. i really appreciate your help.
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