Carnot's theorem

{\begin{aligned}&{}\qquad DG+DH+DF\\&{}=|DG|+|DH|-|DF|\\&{}=R+r\end{aligned}}

In Euclidean geometry, Carnot's theorem states that the sum of the signed distances from the circumcenter D to the sides of an arbitrary triangle ABC is

DF + DG + DH = R + r,\

where r is the inradius and R is the circumradius of the triangle. Here the sign of the distances is taken to be negative if and only if the open line segment DX (X = F, G, H) lies completely outside the triangle. In the diagram, DF is negative and both DG and DH are positive.

The theorem is named after Lazare Carnot (1753–1823). It is used in a proof of the Japanese theorem for concyclic polygons.

External links

Weisstein, Eric W. "Carnot's theorem". MathWorld.
Carnot's Theorem at cut-the-knot
Carnot's Theorem by Chris Boucher. The Wolfram Demonstrations Project.

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