Im bad with latex, otherwise I would draw a graph and show you. i also cant paste my image so im afraid you will just have to use your mind on this one ;P
a sqare has 4 90 degree angles, so when you cut it along a diagonal you will have that each triangle has one 90 degree angle. (these triangles are identitcal) each triangle has to add up to 180 degrees, and we know that one angle is 90 degrees. The other 2 angles in the triangle are the same, so they each have to be 45 degrees.