duct
        of a 1-row by 3-column matrix with a 3-row by 1-column
        matrix: $$dV = \begin{bmatrix}yz&amp;
        ?&amp;?\end{bmatrix}\begin{bmatrix}dx\\dy\\dz\end{bmatrix}.$$
        How do the columns of the first matrix relate to the
        previous portions of the problem.
<p>          The matrix $\begin{bmatrix}yz&amp; ?&amp;?\end{bmatrix}$
          is the <em>derivative</em> of $V$. The columns of this matrix are
          the <em>partial derivatives</em> of $V$.</p>
        
        </li>

