The Two Pillars of Metrical Geometry

There are two really fundamental theorems in metrical geometry. One of them you already know - it is Pythagoras’ theorem. The other one is the Triple quad formula, which you probably don’t know. These two theorems are properly the cornerstones of Euclidean geometry,  spherical geometry and hyperbolic geometry, and once you understand them in the right way, trigonometry becomes much simpler.