Once 
A proof relies on an infinite, limiting process which paves the way to some calculus-like ideas.
A circle is cut up into 6 sectors which are then rearranged into a near rectangle,
What doesn’t look too close to a rectangle at 6 sectors looks better at 26:
As the number of sectors becomes very big the shape becomes indistinguishable from a rectangle and the argument is complete.
I think that Archimedes used such arguments, or, proof by exhaustion, in some of his solid geometry work.
There’s always space on the inter-web for another proof of Pythagoras’s Theorem. Here’s one that uses the following equal areas property of parallelograms.
This kind of area chopping and shape translation is a feature of Euclidean geometry and our senses support it’s veracity at the order of size of the classroom.
The squares on the sides of a right-angle triangle set up a system of parallel lines which can then be used to demonstrate the Theorem using the above equal areas property.
The thread does not stop here though. Taking the parallel line structure which makes this work we get a generalisation of Pythagoras to non-right-angled triangles with the area of the parallelogram on the longest side being the sum of the areas of those constructed on the other two sides.
Conic sections in the sand give shipwrecked Socratic philosopher, Aristippus, good hope.
I wonder if this ancient tale, in which the mathematics of planetary motion some 2000 years prior to it’s application in space travel, is used to signify the presence of intelligent life and therefore hope of salvation has some echo in our modern era.
What obscure and impenetrable modern theorems could be left on a 21st century beach to do the same job?
First definition of triangles from Euclid’s Elements Book I (Fundamentals of Plane Geometry Involving Straight Lines), Definition 20.
Ancient theorem of Euclid (Book III, Proposition 31) still a fundamental part of school maths. The angle in a semi-circle is a right angle.
Enlargement.
Rotations in space make good Saturday morning maths at the King’s School.
In the geometry of mariners, where straight lines are Great Circles, the angle sum of a triangle is more than 180 degrees.
