Because if is invertible then is invertible. Thus, if is not invertible then is not invertible. If is zero matrix then it means is not invertible and so is not invertible.
Because if is invertible then is invertible. Thus, if is not invertible then is not invertible. If is zero matrix then it means is not invertible and so is not invertible.
Thank you TPH, you just answered a question of mine. I'm glad I've searched into the forum.