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A three-sided polygon. The sum of the interior angles of a triangle is always 180, unless the triangle is drawn in a non-Euclidean geometry (see below). Triangles can be classified either by their angles, as acute, obtuse, or right; or by their sides, as scalene (all different), isosceles (two the same), or equilateral (all equal). A right (or right-angled) triangle has one interior angle equal to 90°, and may be either scale or isoscles (see Pythagoras' theorem).

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