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.
A triangle with two equal sides is called an isosceles triangle. The angles opposite the equal sides in an isosceles triangle are also equal. The third, unequal side of an isosceles triangle is called the base of the triangle.