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Homework Help: Proof of difference identities for cosine

  1. Jul 15, 2012 #1

    I am working on proofs of the difference identities for sine, cosine, and tangent.
    I am hoping to solve these using a specific diagram (attached).

    I was wondering if you could help me with the difference of cosines. Is it possible to derive it using the attached diagram? If so, how could I go about this?

    Details and a relevant diagram are attached.


    Attached Files:

  2. jcsd
  3. Jul 16, 2012 #2


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    Science Advisor

    Hey PatternSeeker.

    If you know the difference between the sines, you can use the fact that sin(x) = cos(pi/2 - x) and then get the identities for difference or sum or cosine terms instead of sine terms.

    With the tangent, will need to expand and simplify out sin(a+b)/cos(a+b).
  4. Jul 17, 2012 #3
    Thank you chiro!
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