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Arbitrary triangle

A triangle is isosceles, when two of its sides are of equal length and two angles are congruent. Example of a isosceles triangle

The triangles legs and the base line

The two sides of equal length are also called the triangle’s legs. The third side is called the base line. On the figure to the right the two rays, a and b, coming out from the point C are the legs. The line c is the base line.

Base angles and vertex angle

The base angles are the two congruent angles. The third angle is called the vertex angle.
On the figure A and B are the base angles and C is the vertex area.

The height of an isosceles Triangle

The height from the base line to the vertex divides the vertex angle in two equal angles.
Since the height also divides the base line into two equal parts, the height is a line segment bisector as well as an angle bisector.