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The interior angles of a triangle always add up to 180 degrees. If you make one angle bigger, another angle will become smaller: The exterior angles of a triangle always add up to 360 degrees: In every triangle, the shortest side is always opposite the smallest interior angle. The longest side is always opposite the largest interior angle. âOppositeâ means that the side does not touch the point of the angle on either end. Equilateral triangles must have three angles of 60 degrees each, and all triangles with three 60 degree angles are equilateral.
This rectangle has four 90 degree angles adding up to 360 degrees. Since the triangles are congruent each triangle has half as many degrees, namely 180. So this is true for any right triangle. Suppose you triangle is not a right triangle. Draw a perpendicular from the vertex opposite the longest side, to the longest side.