Euclidean geometry equal (SAS), or two angles and a side equal (ASA) (Book I, propositions 4, 8, and 26). Triangles with three equal angles (AAA) are similar, but not necessarily congruent. Also, triangles with two equal sides and an adjacent angle are not necessarily equal or congruent. The sum of the angles of a triangle is equal to a straight angle (180 degrees). This causes an equilateral triangle to have three interior angles of 60 degrees. Also, it causes every triangle to have at least two acute angles and up to one obtuse or right angle. The celebrated Pythagorean theorem (book I,
Circular triangle Circular triangle In geometry, a circular triangle is a triangle with circular arc edges. A convex circular triangle may be constructed by three circles intersecting each other and represents the area of intersection. Its edges are all curved outwards. The sum of the internal angles of a circular triangle is greater than 180°. A Reuleaux triangle is a special case based on an equilateral triangle where the center of each arc is on the opposite vertex. A circular horn triangle is a similar concept, but represents the area interior to 3 mutually tangent circles so all of the internal angles