1. Let $A = [a_{ij}]$ be an $m \times n$ matrices, $B = [b_{ij}]$ be an $m \times m$ matrices and $\lambda$ be a given constant. Use Sigma notation (i.e. $\Sigma$) and $a_{ij}, b_{ij}$ to represent $\text{trace}(\lambda I - A^T BA)$. (10%)