Triangle Interior -- from Wolfram MathWorld

The interior of the triangle is the set of all points inside a triangle, i.e., the set of all points in the convex hull of the triangle

The Magic of Euler's Equation: V-E+F=2. An Eye Opener., by Peter Sels

Triangle Centroid -- from Wolfram MathWorld

Degenerate Polygons – The Math Doctors

Altitude (Triangle)

PDF) Antiparallels An Overlooked HS Geometry Beauty

A Triangle Median Meet

How to Get an A in Geometry (with Pictures) - wikiHow

2. Art Gallery Theorem

004 Learn Geometry - Interior Angles of a Triangle by Folding - Wolfram Alpha