by 1 unit and $y$ by 1 unit will cause
          $z$ to increase by about ? units.</li>
        </ul>
      </li>

      <li>In the previous part, we said that $z=9-x^2-y^2$
      increased by <em>about</em> a certain amount each time. Why did
      we not say that $z=9-x^2-y^2$ increases by <em>exactly</em>
      that amount?</li>
    </ol>
  </div>We'll conclude this section with a note about taking
  derivatives of higher orders. Since a partial derivative is a
  function, we can take partial derivatives