An equilateral triangle ( Greek: ἰσόπλευρον, romanized: isópleuron, lit.'equal sides') has three sides of the same length.The names used for modern classification are either a direct transliteration of Euclid's Greek or their Latin translations.Īncient Greek mathematician Euclid defined three types of triangle according to the lengths of their sides: The terminology for categorizing triangles is more than two thousand years old, having been defined on the very first page of Euclid's Elements. 10 Specifying the location of a point in a triangleĮuler diagram of types of triangles, using the definition that isosceles triangles have at least 2 equal sides (i.e., equilateral triangles are isosceles).9 Figures circumscribed about a triangle.7.5 Circumcenter, incenter, and orthocenter.7.1 Medians, angle bisectors, perpendicular side bisectors, and altitudes.7 Further formulas for general Euclidean triangles.6.6 Formulas resembling Heron's formula.

5.1 Trigonometric ratios in right triangles.

4 Points, lines, and circles associated with a triangle.

Altitude of a triangle