In geometry, the radius of an excircle (a circle tangent to one side of a triangle and to the extensions of the other two sides).
From Latin ex- 'outside' + radius 'ray' or 'spoke.' A technical geometric term formed in the 18th-19th century as mathematicians formalized circle-triangle relationships.
For every triangle, there are actually four special circles—one inside (incircle) and three outside (excircles)—and if you know the exradii, you can reconstruct the entire triangle from scratch. That's wild!
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