<P> By the Steiner--Lehmus theorem, every triangle with two angle bisectors of equal length is isosceles . </P> <P> The compound is made from "isos" (equal) and skelos (leg). </P> <P> In an isosceles triangle that has exactly two equal sides, the equal sides are called legs and the third side is called the base . The angle included by the legs is called the vertex angle and the angles that have the base as one of their sides are called the base angles . The vertex opposite the base is called the apex . </P> <P> Euclid defined an isosceles triangle as one having exactly two equal sides, but modern treatments prefer to define them as having at least two equal sides, making equilateral triangles (with three equal sides) a special case of isosceles triangles . In the equilateral triangle case, since all sides are equal, any side can be called the base, if needed, and the term leg is not generally used . </P>

Where is the vertex angle of an isosceles triangle