For every non-square matrix prove that $AA^t$ or/and $A^tA$ is singular.
Like the title, I want to prove this and I tried to think of ways to prove it but I couldn't think of some..
I know by this answer that $AA^t$ is symmetric but I cant make the connection.
If someone has at least a hint to that It'll be great if you could write it in the comment section so I could give it a shot!
Thanks in advance.