 values do the maxima
      and minima occur?</li>

      <li>What is $df/dt$ at $t=\pi/4$?</li>

      <li>Compare your work with the previous problem.</li>
    </ol>
  </div>

  <div class="problem">
    Consider again $f(x,y)=9-x^2-y^2$ and $\vec r(t)=(2\cos t,
    3\sin t)$.

    <ol>
      <li>Compute both $Df(x,y)$ and $D\vec r(t)$ as matrices. One
      should have two columns. The other should have one column
      (but two rows).</li>

      <li>The temperature at any time $t$ we can write symbolic