Navigation Home Search Site map document.write (document.title) Contact Graeme Home Email Twitter Math Help ... Equilateral Triangle Napoleon's Theorem Napoleon's theorem: if equilateral triangles are constructed on the sides of any triangle (all outward or all inward), the centers of those equilateral triangles themselves form an equilateral triangle. Proof: Beginning with an arbitrary triangle ABC, construct equilateral triangles a, b, c as shown. Construct a fourth equilateral triangle, d, as shown, with sides equal to those of triangle b. Triangle d is oriented the same as triangle b. (Proof: trace a path along the four line segments consisting of sides of triangles d, a, a, and b. Treating left turns as negative angles and right turns as positive angles, the turns, in sequence, are B-180, 120, and 60-B, which add up to zero.) If both the triangle ABC and the equilateral triangle b are rotated counterclockwise about c through an angle of 120 degrees, side AC will line up with a different side of equilateral triangle c, and so the image of triangle b is concident with triangle d, proving line segments dc and cb are equal in length. Similarly, rotating triangle ABC and equilateral triangle b clockwise through an angle of 120 degrees shows line segments ab and ad are equal in length. By constructing the dark blue line ac, we see it bisects angles a and c, which are each 120 degrees, so angles cab, abc, and bca are all 60 degrees, proving the theorem. Internet references | |
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