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Homework Help: Evaluate a Trigonometric Function

  1. Apr 10, 2010 #1
    1. The problem statement, all variables and given/known data
    1. Given cos α = -24/25 in Quadrant II and sin β = -5/13 in Quadrant III, find sin (α + β).
    2. Given sin α = 4/5 in Quadrant I and cos β = -12/13 in Quadrant II, find cos (α + β).

    2. Relevant equations
    I don't even know where to start! The textbook doesn't list any equations.

    3. The attempt at a solution
    I have none, other than guessing that you multiply the denominators in each problem... other than that, I don't even know how to solve these types of problems. I'm without a teacher, so I'd really like an explanation since I'm a little... really... lost about these problems. Thanks to anyone who can show me how I'd go about solving these! I need to know it for an upcoming final.
  2. jcsd
  3. Apr 10, 2010 #2


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    Science Advisor
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    Gold Member

    Draw pictures of your given angles in the given quadrants with little triangles showing the sides -- you can get the third side by the pythagorean theorem, so you know all the sines and cosines of the given angles. Then use the addition formulas for cos (α + β) and sin (α + β).
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