Pythagoras, Pythagorean, Euclidean, Gravitational Lensing, Symmetry.
90 degrees because the internal angles of any triangle sum 180 degrees: α + β + 90° = 180°, therefore α + β = 90°. If the last sentence is true, then the 4 triangles in the first figure form a square with side “c”, and area A = c². It is also true that inside lies a square with side (b-a) and area