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.
Isosceles Trapezoid. The term isosceles can also refer to the trapezoid, a quadrilateral with a single pair of parallel sides. Much like the triangle, an isosceles trapezoid has a pair of equal non-parallel lines, a single line of reflective symmetry and a pair of matching angles.