Online Geometry

Triangles: Theorems and Problems 40

Triangles

Triangle with the bisectors of the exterior angles

Triangle with the bisectors of the exterior angles. Collinearity. Key concept: Menelaus Theorem.
 

Napoleon's Theorem

Napoleon's Theorem. A purely geometric proof. It uses the Fermat point to prove Napoleon without transformations.
Escalene and Equilateral triangles. 

The Bevan Point The circumcenter of the excentral triangle. Illustration with animation.

The Bevan Point: Puzzle

Angle between two Simson Lines. Proof with animation.
 

Simson Line. A proof of Simson line with animation.

Butterfly Theorem Proof with animation.

Butterfly Theorem: Puzzle.

Langley's Problem Adventitious angles. 20° isosceles triangle with animation.
 

Triangle with Squares Problem

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