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Any formulas for arctan(a/b)?

  1. Feb 25, 2012 #1
    Hey, I'm wondering if anyone knows of a trig identity for arct(a/b) where a and b are rationals.
  2. jcsd
  3. Feb 26, 2012 #2


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  4. Feb 26, 2012 #3
    Thanks for the reply. Although I can't find arctan(a/b), this is still very helpful for what I'm doing.
  5. Feb 26, 2012 #4
    Hi, perhaps you can tell us what you're doing. That would provide a context to work in.

    In general, what's an arctan? If you have a line through the origin in the x-y plane, say it goes through some nonzero point (b,a). Then its slope is a/b, to be consistent with your notation.

    The slope of the line is the tangent of the angle the line makes with the positive x-axis taken counter-clockwise. So the arctan of a/b is just the angle made by a line that passes through the origin and the point (b,a).

    I don't know whether that's helpful or not, since I don't understand what you're trying to do. But it's one way to think about the arctan function, especially if you're already given the argument as a quotient (of two rationals or two reals, doesn't matter).

    Another way to think about the arctan is to convert a complex number from rectangular to polar form. Given z = b + ai, to convert to polar form you end up taking the arctan of a/b (again reversing the usual use of a and b to conform with your notation).

    And still a third way to think of the arctan is that it provides a handy gadget any time you need to continuously biject the entire real line onto a bounded open interval.

    Perhaps one or more of these points of view will provide you with some insight into your problem.
    Last edited: Feb 26, 2012
  6. Feb 26, 2012 #5
    I wanted to show that arctan(a/b) may be written in the form arctan(a*m)+arctan(b*n) (or something like that) as part of a proof Im writing for a project. The entire explanation is long winded and it would take some time to explain but basically if I know that (in my project) all arctan(a) and arctan(b) and any linear combination of those exist but I have yet to show if all arctan(a/b) exist or not which is why I was hoping for a trig identity that would neatly answer the question
  7. Feb 26, 2012 #6
    Well, arctan is defined for all real numbers. So no matter what a and b are, arctan(a/b) exists. Am I misunderstanding?
  8. Feb 26, 2012 #7
    But i want it to exist in the constraint of my problem. In my problem, all arctan(a) and arctan(b) exist but I dont know if all arctan(a/b) exist in my problem. For example, I know that arctan(I) where I is an irrational number does not exist in my problem
  9. Feb 26, 2012 #8


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    (aside: a/b isn't defined for all real numbers....)
  10. Feb 27, 2012 #9
  11. Feb 28, 2012 #10
    oo thanks Guffel!
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