An
isosceles triangle is a
triangle with at least two equal
sides, meaning
that the lengths of those sides are equal. A new set of terms accompanies the
isosceles triangle. The equal sides are called
legs. The
third side is the
base. The
angle opposite the
base is the
vertex angle. The angles
opposite the legs are called the
base angles.
Figure 2.1: A generic isosceles triangle
Sides
a and
b are the legs. Side
c is the
base. Angle C is the vertex angle. Angles A and B are the base angles.
The beauty of an isosceles triangle is that, just like the sides, the base
angles are equal. From this we learn that if two angles of a triangle are
equal, the sides opposite them are also equal, and the triangle is an isosceles
triangle. We also know the converse: if two sides of a triangle are equal,
their opposite angles are equal, and the triangle is isosceles.
