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Homework Help: Another Differential Equations Problem

  1. Oct 4, 2008 #1
    1. The problem statement, all variables and given/known data
    For what values of K does the function y=cos k t satisft the differential equation 4y"=-25y?

    Do I have to find the derivative of y twice or what do i do
  2. jcsd
  3. Oct 4, 2008 #2
    ... what does y'' mean?
  4. Oct 4, 2008 #3
    it means second order derivative I believe
  5. Oct 4, 2008 #4
    Yes, find the second derivative and compare constants.
  6. Oct 4, 2008 #5
    ok so it would be y"=-k^2cos kt
  7. Oct 4, 2008 #6
  8. Oct 4, 2008 #7
    then do i plug in values for it
  9. Oct 4, 2008 #8

    Gib Z

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    Homework Helper

    You have 4y"=-25y. Substitute what you know to be y and y'', and equate coefficients as Tedjn said.
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