Comment by SJC_Hacker

> The purely trigonometric proof that was found by some high school students recently is a great one.

I failed to understand what was so cool about that proof. It relied on concepts such as Cartesian coordinate systems, and the measure of an angle (not just a pure geometric concept), and even concepts like convergence of infinite sums, which weren't purely geometric.

Geometry had been formalized in the 20th century and had moved past informal proofs