The most difficult mathematics is that which you do not know. A surprising amount of mathematics is actually easy once you've learned it. Of course, once you learn the easy stuff, then you have to start tacking the deep stuff, and that gets harder.
An isosceles triangle has two sides of equal length. An isosceles triangle also has two angles of the same measure; namely, the angles opposite to the two sides of the same length; this fact is the content of the isosceles triangle theorem, which was known by Euclid.
Recall that an isosceles triangle is a triangle with at least two congruent sides. These congruent sides are called legs. The point at which these legs meet is called the vertex point of the isosceles triangle, and the angle formed by the legs is called the vertex angle. The other two angles of the triangle are called base angles. A labeled illustration of an isosceles triangle is shown below. In the past, we determined isosceles triangles by the lengths of their sides. In other words, if we saw that a triangle had two sides with equal lengths, then we classified the triangle as isosceles. There are other characteristics that mark isosceles triangles, however.