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.
Two triangles can be right isosceles triangles but they do not necessarily have to be congruent. For example, let us say, Right Isosceles Triangles ABC and DEF with the right angles at C and F, respectively. Now, side a and side b must be equal since ABC is isosceles.