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Homework Help: Triangle Circumscribed by a Circle (Geometry Problem)

  1. Feb 19, 2009 #1
    1. The problem statement, all variables and given/known data

    Alright... so I thought of this problem on the train ride home. I am sure plenty thought of it before me...but now it is my turn.

    Say want to know what the length L an equilateral triangle must have such that all three points touch the circle of radius R that it is contained by.

    I have drawn the problem. i have bisected one of the 60 degree angles to form a right triangle.

    I then drew a line perpendicular to the bisecting line to create a similar triangle.

    I do not know if this is the way you would have approached this, but it seemed pretty logical to me.

    I feel like I have almost all (if not all) the necessary info to write L in terms of R.

    Can someone chime in with a hint here?

    Picture1-25.png --->


    I feel like I need something more about either the short leg of the inner triangle or its hypoteneuse. then I can relate
    both triangles to the angle.
  2. jcsd
  3. Feb 19, 2009 #2


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    You just want the relation between L and R? I see in your picture a triangle with a 120 degree angle at the center between two legs of length R and with an opposite side of length L. It's just trig, saladsamurai, I know you know that. It must have been a rough train ride home.
  4. Feb 19, 2009 #3
    I presume this is what you meant Dick:


    I can honestly say that I may have never seen that if you did not point it out :/

    I have never taken a geometry class in my life, so these things do not pop out at me sometimes... arggggghhhh

    Thank Dick!
  5. Feb 19, 2009 #4


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    You didn't even need to see it. I also see a 30-60-90 right triangle with a hypotenuse of R and a leg of length L/2. I'm still blaming the train ride home.
  6. Feb 19, 2009 #5
    Now you're just showing off after I said I suck at Geometry!! I STILL do not see that one!

    edit: Okay NOW I see it! But I never would have since I still would have needed to have drawn that same line that allowed me
    to see the 120 degree triangle.
  7. Feb 20, 2009 #6


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    Yeah. You would have gotten it just fine if you'd drawn another radius line. Remember that when you're stuck in the future.
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