Trigonometry/Circles and Triangles/Brocard's Theorem
Brocard's Theorem is due to French mathematician Henri Brocard (1845 – 1922).
Let ABC be any triangle. Draw three lines:
- AD where D is between B and C, and angle DAB = ω
- BE where E is between A and C, and angle EBC = ω
- CF where F is between A and B, and angle FCA = ω
Then the lines AD, BE, CF are concurrent, meeting at a Brocard point, if and only if
- cot(ω) = cot(A) + cot(B) + cot(C).
From symmetry, there is a second Brocard point, using the same angle ω, at the intersection of the three lines
- AD' where D' is between B and C, and angle D'AC = ω
- BE' where E' is between A and C, and angle E'BA = ω
- CF' where F' is between A and B, and angle F'CB = ω