Three Touching Circles on a Line

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The radius of the middle circle (purple) of three circles all touching each other and a line is given by a formula in terms of the radii of the two outside circles. Drag the points to vary the outside circles.
Contributed by: Noel Patson (March 2011)
Open content licensed under CC BY-NC-SA
Snapshots
Details
Idea taken from Sangaku.
Permanent Citation
"Three Touching Circles on a Line"
http://demonstrations.wolfram.com/ThreeTouchingCirclesOnALine/
Wolfram Demonstrations Project
Published: March 7 2011