Proposition from Liber Quadratorum Prove the following proposition: For any odd square number $x$, there is an even square number $y$, such that $x + y$ is a square number

Think of these as pythagorean triples. For every positive integer n:

$$ (4n+1) + (2n)^2 = (2n+1)^2 $$

So all we need to do is show all odd squares are of the form $4n+1$ for some n, and we are done. This is perhaps most easily done by showing that no odd number of the form $4n+3$ can be a square number (the contrapositive).

Hence, given an odd square x, one can write it in the form $4k+1$ for some k, and the even square you want, y, is simply $(2k)^2$.

Edit: Henry's answer was posted as I was writing this and I possibly got a bit carried away with detail as it was my first answer on the site, a hint might have been better.