Pythagoras, Pythagorean, Euclidean, Gravitational Lensing, Symmetry.
to do directly with the Euclidean (yes, made by Euclid) postulates. You can read about the postulates and their relationship with triangles internal angles, or take a look at my explanation on the next figure: Figure 5 For simplicity, I will just treat the case of a right triangle here. In the