1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar

    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

    User Avatar

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Is this right? (trig equation)
  1. Trig equation (Replies: 4)

  2. Trig. Equation (Replies: 8)

  3. Trig equation (Replies: 10)

Loading...