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Triangles are three-sided polygons, but we're sure that you already knew that. The triangle is the most sturdy of polygons. Its strong shape has been used to build buildings and bridges since the dawn of civilization. It has so much more muscle than a (wimpy) square. Think about it; that's why there are three legs on a tripod and three wheels on a tricycle.
Triangles can be classified (named) two ways: by their angles or by their sides. Here they are classified by angles:
Here they are classified by sides (when the markings on the sides are the same, that means that the sides are congruent):
Here they are classified by both their angles and sides:
The interior angles of a triangle always add up to 180°, always. Since regular polygons have equal sides and angles, all equilateral triangles are regular. If we divide 180° into three angles, each angle of an equilateral triangle is 180 ÷ 3 = 60°.
Here's a video on interior and exterior angles, just to mix things up a little.