ally
      as $f(r(t))$. First semester calculus suggests the derivative
      should be the produce $(f(\vec r(t))) ' = f'(\vec r(t))\vec
      r'(t)$. Write this using $D$ notation instead of prime
      notation.</li>

      <li>Compute the matrix product $DfD\vec r$, and then
      substitute $x=2\cos t$ and $y=3\sin t$.</li>

      <li>What is the change in temperature with respect to time at
      $t=\pi/4$? Is it positive or negative? Compare with the
      previous problem.</li>
    </ol>
  </div>Th