Acute and obtuse triangles "x", then If two obtuse triangles have sides ("a, b, c") and ("p, q, r") with "c" and "r" being the respective longest sides, then The Calabi triangle, which is the only non-equilateral triangle for which the largest square that fits in the interior can be positioned in any of three different ways, is obtuse and isosceles with base angles 39.1320261...° and third angle 101.7359477...°. The equilateral triangle, with three 60° angles, is acute. The Morley triangle, formed from any triangle by the intersections of its adjacent angle trisectors, is equilateral and hence acute. The golden triangle is the isosceles
Exterior angle theorem segments). Only one of these angles contains the third side of the triangle in its interior, and this angle is called an "interior angle" of the triangle. In the picture below, the angles "∠ABC", "∠BCA" and "∠CAB" are the three interior angles of the triangle. An "exterior angle" is formed by extending one of the sides of the triangle; the angle between the extended side and the other side is the exterior angle. In the picture, angle "∠ACD" is an exterior angle. The proof of Proposition 1.16 given by Euclid is often cited as one place where Euclid gives a