"That which was to be proved has been proved."
Do they teach mathematical/geometrical proofs as part of the National Curriculum these days ? It's a bit like how you prove, logically, that the square on the hypotenuse is equal to the sum of the squares of the other two sides, for example. If you learned maths in a classical education in the 70s, at the end of it you got one extra mark for remembering to write Q.E.D. - Quod Erat Demonstrandum.