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Basic geometry question re isosceles triangles

  1. Nov 13, 2012 #1
    I am reading "Mathematical Methods for Scientists and Engineers" by Donald McQuarrie. In his discussion of polar coordinates, he uses a geometric argument to derive the differential area element, which is of course rdrdθ. He shows an isosceles triangle whose two equal sides are r, and the angle that they make at the top of the triangle is delta(theta), where delta(theta) is small. From this he says it follows that the base of the triangle is equal to
    However, I can not find this in my geometry book. Shouldn't the base be something like
    I also ran into this same problem in an astrophysics book where the author used the very same argument for deriving the formula for the distance to a star by determining the parallax angle.
    In both cases, are the authors using the "small angle approximation" that sin(theta) = theta?
    Sorry to ask such a basic geometry question. I'm 61 years old, and I don't remember my high school geometry that well.
  2. jcsd
  3. Nov 13, 2012 #2
    Yes, they are using the small angle approximation that you refer to. Actually, the exact value would be 2rsin(delta/2), which comes out to r *delta using that approximation. These linear approximations are valid in integration theory because of the nature of the integral. It is defined as a limit of a sum as you partition the region of integration into small pieces. So on each small piece, the linear approximation is quite accurate. In the limit, (as the partition of the region gets finer and finer) the linear approximation is exactly correct.
  4. Nov 13, 2012 #3


    Staff: Mentor

    What you have written doesn't make much sense - sin by itself is as meaningful as √ by itself.

    I don't have the book you cite, but I'm going to guess that he's approximating the area of part of a circular sector. This region is four-sided, with two of the sides being curved.

    If you take what you're calling an isosceles triangle (actually it's a sector of a circle), and extend the two rays by Δr, you get the four-sided figure. It's roughly a rectangle, where one dimension is Δr and the other is rΔθ. If you multiply those, you get rΔθΔr, which is the same as rΔrΔθ.

    The part that might be confusing you is the rΔθ. This is the length of the arc of a circle of radius r, that is subtended by an angle of measure Δθ. If the circle's radius is 1, the arc length will be 1Δθ or Δθ. If the circle's radius is 2, the arc length will be 2Δθ. In general, if the radius is r, the arc length is rΔθ.
    It might not have been covered. When I took geometry (a few years before you did), most of the time we studied triangles, and didn't do much with other geometric figures.
  5. Dec 5, 2012 #4
    Thank you.
    Holiday Greetings!
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