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Homework Help: Is this right? (trig equation)

  1. Jun 12, 2006 #1
    It's set up like this and you have to solve for all possible angles:

    -360 degrees <= theta <= 360 degrees

    8cos ^ 2 theta + 7cos theta - 1 = 0

    (my work)

    (8cos - 1)(cos theta + 1) = 0

    8cos theta - 1 = 0
    8cos theta = 1
    cos theta = 1/8
    theta = 1/8 cos ^ -1

    theta = 83 degrees
    = -83 degrees
    = 277 degrees
    = -277 degrees

    OR

    cos theta + 1 = 0
    cos theta = -1
    theta = -1cos ^ -1

    theta = 180 degrees
    = -180 degrees

    The back of the textbook has only the answers before my "OR", they do not show the possible 180 and -180 degrees that I calculated, did they just forget or did I mess something up?
     
  2. jcsd
  3. Jun 12, 2006 #2

    Doc Al

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    Staff: Mentor

    They must have left them out--your answers are correct. You can verify that those answers are correct by plugging them back into the original equation.
     
  4. Jun 12, 2006 #3
    Ok thanks for your time, it not's the first time I've encountered incorrect(or in this case exclusive) answers in a textbook, just wanted to make sure.
     
  5. Jun 12, 2006 #4
    Actually there's one more, would you mind helping me?
     
  6. Jun 12, 2006 #5

    Doc Al

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    Staff: Mentor

    Let's have it. :smile:
     
  7. Jun 12, 2006 #6
    Never mind I just figured it out, I accidentally wrote 4/2 * 1 = 1 instead of 2 :surprised

    I got the right answer after that correction, thanks anyways.
     
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