Formulation of the theorem
Note that in the degenerate case, where all four circles reduce to points, this is exactly Ptolemy's theorem.
The following proof is due to Zacharias. Denote the radius of circle by and its tangency point with the circle by . We will use the notation for the centers of the circles. Note that from Pythagorean theorem,
We will try to express this length in terms of the points . By the law of cosines in triangle ,
Since the circles tangent to each other:
Let be a point on the circle . According to the law of sines in triangle :
and substituting these in the formula above:
And finally, the length we seek is
It can be seen that the four circles need not lie inside the big circle. In fact, they may be tangent to it from the outside as well. In that case, the following change should be made:
If are both tangent from the same side of (both in or both out), is the length of the exterior common tangent.
If are tangent from different sides of (one in and one out), is the length of the interior common tangent.
The converse of Casey's theorem is also true. That is, if equality holds, the circles are tangent to a common circle.
- Casey, J. (1866). "On the Equations and Properties: (1) of the System of Circles Touching Three Circles in a Plane; (2) of the System of Spheres Touching Four Spheres in Space; (3) of the System of Circles Touching Three Circles on a Sphere; (4) of the System of Conics Inscribed to a Conic, and Touching Three Inscribed Conics in a Plane". Proceedings of the Royal Irish Academy 9: 396–423. JSTOR 20488927.
- Bottema, O. (1944). Hoofdstukken uit de Elementaire Meetkunde. (translation by Reinie Erné as Topics in Elementary Geometry, Springer 2008, of the second extended edition published by Epsilon-Uitgaven 1987).
- Zacharias, M. (1942). Jahresbericht der Deutschen Mathematiker-Vereinigung 52. Missing or empty
- Johnson, Roger A. (1929). Modern Geometry. Houghton Mifflin, Boston (republished facsimile by Dover 1960, 2007 as Advanced Euclidean Geometry).
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