## Sunday, February 10, 2013

### Strange Isoperimetry

Here we see a triangle constructed with all three sides tangent to an external circle. Because the solid and dotted blue segments have a common point and are bounded by tangencies to a common circle, they are the same length; similarly, the solid and dotted red lines have the same length; therefore the perimeter of the triangle is equal to the sum of the segments between the apex (of cyan and magenta) and the external tangencies --- which two segments are also equal; in particular, the perimeter is determined by the apex angle and the circle radius.

By way of a converse, the envelope of triangles within fixed rays (as the cyan and the magenta, continued away from the apex) and having a fixed perimeter is a circle arc tangent to those fixed rays at a distance of half the fixed perimeter from their intersection.