The Six-Point Circle Theorem
Given $ΔABC$ and angles $α,β,γ\in(0,π)$ with $α+β+γ=π$, we study the properties of the triangle $DEF$ which satisfies: (i) $D\in BC$, $E\in AC$, $F\in AB$, (ii) $\aangle D=α$, $\aangle E=β$, $\aangle F=γ$, (iii) $ΔDEF$ has the minimal area in the class of triangles satisfying (i) and (ii). In particular, we show that minimizer $ΔDEF$, exists, is unique and is a pedal triangle, corresponding to a certain pedal point $P$. Permuting the roles played by the angles $α,β,γ$ in (ii), yields a total of six such area-minimizing triangles, which are pedal relative to six pedal points, say, $P_1,....,P_6$. The main result of the paper is the fact that there exists a circle which contains all six points.
math.MG↗