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Trig identity proof.

  1. Aug 18, 2012 #1
    Prove:
    [itex]\frac{CosθSinθ}{1 + Tanθ}[/itex] = Cos2θ
    ===========================
    I multiply out the denominator to get:

    CosθSinθ = Cos2θ + CosθSinθ

    I cannot seem to prove it.

    Starting to think it's a trick question.. :/
     
  2. jcsd
  3. Aug 18, 2012 #2
    It most certainly is a trick question because the supposed identity doesn't even work if you plug in theta = 0. The left side is 0 while the right is 1, in this case!
     
  4. Aug 18, 2012 #3
    You might be asked to solve the equation instead of proving the identity, and the solutions of the equation are the zeros of cosine, id est odd integer multiples of [itex]\pi/2[/itex].
     
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