My favorite theorem is the Pythagoras' theorem. The summary for the theorem is
"The square of the hypotenuse of a right triangle is equal to the sum of of the squares on the two sides."
x = side A y = side B z = hypotenuse
z^2 = x^2 + y^2
This is one of my favorite because it is simple and beautiful. This is one of the earliest theorem that we would learn and it is very important. This theorem is applied virtually everywhere especially in architectures, mechanical designs and etc.
I am also aware that a lot of other people chose this theorem as their favorite, because it is also one of the easiest theorem to remember in their head. And it is one of the first theorems that we learned in our lifetime.
