## Proving rank of a matrix

Hi.

Given X E R^(n x k) and X has linearly independent columns, and A = X(X'X)^-1 X', I want to prove rank(A) = k.

I was told to show im(A) = L(X).

If i do that it would make sense since the Linear span of X is the basis of X with l.i columns, and dimL(X) = rank(X).

How do i show images though. I'm really unsure at that point.