Pythagoras, Pythagorean, Euclidean, Gravitational Lensing, Symmetry.
3 colored copies of the red triangle and arranging them in the way shown in Figure 1, unleashes the hidden answer. You can do this, no matter the shape of the original triangle, as long as it has a 90 degrees angle. If you are not convinced, watch Figure 4, or realize that alpha and beta always sum