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Homework Help: Converting one inverse trig function to another

  1. May 31, 2016 #1
    1. The problem statement, all variables and given/known data
    Express $$ 2 arctan (\sqrt\frac{a-b}{a+b} tan (\theta/2))$$ in terms of inverse cosine

    2. Relevant equations
    I realise it amounts to find a smart substitution, but I can't find one.
    3. The attempt at a solution
    I tried ##b/a=tan \theta## , but I can't find any way to get rid of the other tangent term. I would be really nice if the solution is not a direct substitution of the enormous formula for the inverse trig functions, as it only gets messy. Thanks in advance!!
  2. jcsd
  3. May 31, 2016 #2


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  4. May 31, 2016 #3
    @Math_QED I mentioned that I don't want to just substitute into the formulae as they are very messy...No problem, you can look into it when you have the time.
  5. May 31, 2016 #4


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    Hint: Let ##x = 2 \arctan \left[\sqrt\frac{a-b}{a+b} \tan \left(\frac \theta 2\right)\right]##. Then ##\tan \frac x2 = \sqrt\frac{a-b}{a+b} \tan \left(\frac \theta 2\right)##. You can express ##\tan\frac x2## in terms of ##\cos x##.
  6. May 31, 2016 #5
    Done!! Thanks a lot @vela !!!
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