Some basic theorems about similar triangles are: The corresponding sides of similar triangles have lengths that are in the same proportion, and this property is also sufficient to establish similarity. Two triangles are said to be similar, if every angle of one triangle has the same measure as the corresponding angle in the other triangle. The sum of the measures of the three exterior angles (one for each vertex) of any triangle is 360 degrees. The measure of an exterior angle of a triangle is equal to the sum of the measures of the two interior angles that are not adjacent to it this is the exterior angle theorem. An exterior angle of a triangle is an angle that is a linear pair (and hence supplementary) to an interior angle. This allows determination of the measure of the third angle of any triangle, given the measure of two angles. This fact is equivalent to Euclid's parallel postulate.
The sum of the measures of the interior angles of a triangle in Euclidean space is always 180 degrees. The measures of the interior angles of the triangle always add up to 180 degrees (same color to point out they are equal). Elementary facts about triangles were presented by Euclid, in books 1–4 of his Elements, written around 300 BC.
In rigorous treatments, a triangle is therefore called a 2- simplex (see also Polytope). Triangles are assumed to be two- dimensional plane figures, unless the context provides otherwise (see § Non-planar triangles, below). A triangle with vertices A, B, and C is denoted △ A B C īasic facts A triangle, showing exterior angle d. It is one of the basic shapes in geometry. A triangle is a polygon with three edges and three vertices.
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