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Homework Help: Simplifying multiple trig functions into a single trig function for physics II

  1. Jan 19, 2012 #1
    1. The problem statement, all variables and given/known data

    It has been a while since I have really been involved in trig seriously, But I felt it appropriate to go in this particular forum because in my classes from years back "precal" was the title associated with trig (:

    The Problem:

    sin(X) / sin(X/2) ----->Somehow simplifies into-----> 2*cos(x/2)

    I realize this is probably a basic skill I should have, but until this problem I have never really had issues with any of the math in my physics classes, But perhaps I am just looking in the wrong places for the "How" of this situation.

    2. Relevant equations

    I am 100% sure of the accuracy of this relation, since it was derived from wolfram and then used to answer a refraction physics question which came back correct. I would just like to know where I can look so I can learn how this simplification is possible that I may do this on my own without help.


    3. The attempt at a solution

    I tried looking at relevant trig Identities...but none of them seem to apply here..seems there is some *tricky* stuff here as my calculus teachers always said haha d:
  2. jcsd
  3. Jan 19, 2012 #2


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    Homework Helper


    Covers the double-angle formulae. From these, the half-angle formulae can be simply derived (can you see how?). Scroll down till you see the half-angle formula for sine.
  4. Jan 19, 2012 #3
    you would need to do some algebraic manipulation. I'll start it out for you...

    2cos(x/2) = 2 * ((1 + cosx)/2)^1/2 (half angle identity)
    = 4(1+cosx)/2 by squaring.

    Then multiply top and bottom by (1-cosx) and reduce to get

    (2(1-(cos x)^2) / (1-cosx)

    You will eventually get (sinx)^2/(1/2-1/2cosx) and then you will take the root of top and bottom and use the half angle identity to arrive at your answer.
  5. Jan 19, 2012 #4


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    Homework Helper

    You can do this a lot of ways, but probably the easiest is to write sin(x)=sin(2*(x/2)). Then use sin(2*a)=2*sin(a)*cos(a).
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