Problem Problem 246. Napoleon's Theorem I. External Equilateral triangles. Outer triangle. Level: High School, College, SAT Prep.

In the figure, equilateral triangles ABC', BCA', and ACB' are constructed externally on the sides of a triangle ABC. Prove that the centers A₁, B₁, and C₁ of those equilateral triangles form an equilateral triangle, called the outer Napoleon triangle. Post a comment.