Bethan Copeland's theorem

if you start with a square number,
add the next square number,
then subtract the next square number,
you always end up with a number that's 4 away from the square number before the one you started with

e.g. 16 + 25 - 36 = 5 and 9 - 4 = 5



Bethan discovered and proved this when in y10