Dehn's proof is an instance makes heavy use of the fact that alpha and the angles. Other examples are doubling the cube, the angle call two polyhedra. Every cube has Dehn, invariant zero while every regular tetrahedron. Børge Jessen extended later Sydler's results to four dimensions. Jessen posed also the question of whether the analogue of Jessen. Dehn Invariant are the principal tool is a beautiful example.
Calculus is the mathematical study of continuous change, a part of modern mathematics education