my favorite theorem is Kepler's third law.
in short, he discovered and explained Planetary Motion using his theorem. which is
1. The orbit of all planet is an ellipse with the sun at one of the foci.
2. every planet having their orbit sweep out same areas while equal period.
3. if we set P as a period of revolution and set A as a radius of orbit from a foci then,
$ P^2 = A^3 $
