1. The problem statement, all variables and given/known data If A is an mXn matrix, B is an nXm matirx, and n<m, then AB is not invertible. 2. Relevant equations 3. The attempt at a solution By doing A is a 2X1 and B is a 1X2, I find that AB is not linearly independent, so it cannot be invertible, but I'm not sure how to show that for all matrices of this nature.