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This allows determination of the third angle of any triangle as soon as two angles are known.
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The internal angles of a triangle in Euclidean space always add up to 180 degrees. Elementary facts about triangles were presented by Euclid in books 1–4 of his Elements, around 300 BCE. 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).
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