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.
The easiest way to define an isosceles triangle is that it has two equal sides. In an isosceles triangle, we have two sides called the legs and the remaining side called the base. The angle located opposite the base is called the vertex. Sample A: The vertex angle B of isosceles triangle ABC is 120 degrees.