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Homework Help: Identities of cos

  1. Nov 25, 2005 #1
    i and the rest of my class are finding it impossible to find the answer to this part of the question:

    find the exact value of cos A from secA-cosA=1/2

    thanks in advance
  2. jcsd
  3. Nov 25, 2005 #2


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    Make the substitution [itex] \cos A =x [/itex], then the definition of secant and in the end you should get a quadratic equation in "x".

  4. Nov 25, 2005 #3
    What you do is that you multiply everything by cosA, so that you get a quadratic, and then solve that in terms of cosA, to get A, simply inverse cos it, eg

    (1/cosA) - cosA = 1/2
    -cos^2A + 1 = (1/2)cosA
    0 = cos^2A + 0.5cosA -1
    Then solve with the quadratic formula:biggrin:

    edit: dam,beaten to it...
  5. Nov 25, 2005 #4
    merci beaucoup
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