Can anyone help me solve these? I'm totally stuck...

1. Let A be invertible. Prove that the transpose of A (denoted $\displaystyle A^t$)

is invertible and $\displaystyle (A^t)^{-1}=(A^{-1})^t$

2. Prove that if A is invertible and $\displaystyle AB=0$, then $\displaystyle B=0$.

Any help on these?