ith respect to
    $y$. We'll write this as $\frac{\partial f}{\partial y} =
    x^2+20\cos(5y) $ or $f_y = x^2+20\cos(5y)$.</li>

    <li>Remember, the derivative of $f$ is a matrix. The columns of
    the matrix are the partial derivatives with respect to the
    corresponding input variables&mdash;the first column is the partial
    derivative with respect to the first input variable, the second column with respect to the second input variable, etc.</li>
  </ul>
<p>Now we generalize the above example.</p