6 (10) Let $A, B$ and $D$ be real matrices. Suppose that $A = A^\top$, $D = D^\top$ and $A$ is invertible. Then show that $\left[ \begin{array}{cc} A & B \\ B^\top & D \end{array} \right]$ is positive if and only if $A$ and $D - B^\top A^{-1}B$ are positive definite.