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.
Find the third angle of an isosceles triangle. Isosceles triangles have two equal sides and two equal angles. The equal sides are marked by one hash mark on each of them, indicating that the angles across from each side are equal.