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How can I get the length of the red line, if I got the diameters of all black circles? I'd prefer to get the lengths of the right example but I think it's much more difficult.

https://dl.dropbox.com/u/62849616/Stackoverflow/diameter.png

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    The left one is called outer Soddy circle, see [here](http://gogeometry.com/geometry/soddy_descartes_circles.htm) or [here](http://mathworld.wolfram.com/SoddyCircles.html).2012-12-02
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    yes the first one but id like to get the right one, too.2012-12-02
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    Can you provide some context? Do you need symbolic formula (it will be rather complex)?2012-12-02
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    context: https://dl.dropbox.com/u/62849616/IMAG0374.jpg https://dl.dropbox.com/u/62849616/IMAG0372.jpg2012-12-02
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    Well, then the easiest solution would be to use a measuring tape or some string (I know it is obvious). The reason being, for the formula to give the correct results you need the diameters of inner circles measured with high accuracy and it might be hard in this case.2012-12-02

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