Iff Converse Proof Sorter

Age 14 to 18
Challenge Level

This proof sorter is for the problem Iff.

Can you show that if $8k+1$ is a square number then $k$ must be a triangular number?

You can use the interactive proof sorter below to help you contruct a proof, or if you prefer here is a set of proof sorter cards for you to print out and rearrange.