The midpoint of the hypotenuse is (a,b)...and the easiest way to prove this is:
if the co-ordinates of a line segment are (a,b) and (c,d), the co-ordinates of the midpoint are [(a+c)/2,(b+d)/2]
Also the so-called side-splitter theorem will give you the same result.
Then there are 4 congruent right triangles...draw a figure and you will immediately see this.