Triangle their internal angles, measured here in degrees. A triangle that has two angles with the same measure also has two sides with the same length, and therefore it is an isosceles triangle. It follows that in a triangle where all angles have the same measure, all three sides have the same length, and such a triangle is therefore equilateral. Triangles are assumed to be two-dimensional plane figures, unless the context provides otherwise (see Non-planar triangles, below). In rigorous treatments, a triangle is therefore called a "2-simplex" (see also Polytope). Elementary facts about triangles were presented by Euclid in books 1–4
Automedian triangle sharing no common factor) as with formula_4 and formula_5 coprime, formula_6 odd, and to satisfy the triangle inequality formula_7 (if the quantity inside the absolute value signs is negative) or formula_8 (if that quantity is positive). Then this triangle’s medians formula_9 are found by using the above expressions for its sides in the general formula for medians: where the second equation in each case reflects the automedian feature formula_11 From this can be seen the similarity relationships There is a primitive integer-sided automedian triangle that is not generated from a right triangle: namely, the equilateral triangle with sides of unit