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Trigonometric Identities Proof

  1. Aug 15, 2009 #1
    1. (A) sin(-x) = - sin x (C) cos(x+y) = cosxcosy - sinxsiny
    (B) cos(-x) = - cos x (D) sin(x+y) = sinxcosy + cosxsiny
    Use these equalities to derive the following trigonometric identities.
    a. absolute value of cos x/2 = [tex]\sqrt{}1 + cosx/2[/tex]
    b. absolute value of sin x/2 = [tex]\sqrt{}1 - cosx/2[/tex]



    2.above



    3. I'm stuck on these two and tried to think of different ways to solve it but I can't seem to get find a solution to it. Can you please explain how to derive those two equations? Thank You!!
     
  2. jcsd
  3. Aug 15, 2009 #2

    rock.freak667

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    cos(A+B) = cosAcosB - sinAsinB, try setting A=B and then see what happens. Since there is an x/2, maybe you should replace A with that.
     
  4. Aug 15, 2009 #3
    (B) cos(-x) = - cos x
    needs to be cos(-x) = cos x without the minus in front on the right side.
     
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