Help with Rank and Nullity of transpose matrices I'm stuck on this question;
Show that $Nul(A^tA) = Nul(A)$ for every matrix A.
I don't really know where to start on this. I know that rank is not changed by transposing, so nullity is also something I can figure out (rank nullity theorem).
But this is as far as I can get, and I have no idea how to tackle this in regards particularly to the matrix product, $A^tA$.
EDIT: I basically just added everything I know about the topic. Question doesn't change.

In case you are working in linear algebra (as your tag specifies) you can think that the Null Space of a linear map (or application) represented by $A$ are all those vectors $x$ of the Domain such that:
$Ax = 0$
And $Ax = 0$ iff $x^TA^T = 0$. Then 
$x^TA^TAx = 0$ will be true for exactly the same $x$ for which $Ax = 0$ is true...
