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Laws of Sine/Cosine

  1. Oct 14, 2006 #1
    Law of Sine = Sin A/a = Sin B /b = Sin c/c

    Law of Cosines
    a^2=b^2 + c^2 - 2bc Cos A

    I used to know how to "prove" both of these laws but I haven't touched math for years.

    Can someone "prove" them or "explain" what these laws exactly mean?
  2. jcsd
  3. Oct 14, 2006 #2


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    As for the law of sines:
    Use the sine law for the AREA of triangles in a clever way!

    As for the cosine law:
    Make a triangle with acute angles, draw its height h, so that you have two right-angled triangles.
    Fiddle around with the Pythogorean theorem until you stumble upon the law of cosine
  4. Oct 14, 2006 #3
    Sorry, I don't understand either. I been away from math too long. I know I could just use these formulas but I want to see the "proof".

    Is it possible you could show me an example of proving them?
  5. Oct 16, 2006 #4
    My friend taught me this...dont' quite remember, i would appreciate any help :D
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