
      <li>As the horse moves past the point at
      $t=\pi/4$, is the temperature of the surrounding air
      increasing or decreasing? In other words, is $\dfrac{df}{dt}$
      positive or negative? Use your graph to explain.</li>

      <li>We'll complete this part in class, but you're welcome to
      give it a try yourself. <div class="instructor">See <a href=
      "https://sagecell.sagemath.org/?z=eJyFj0FuwjAQRfc-RXaMk0kV2WLBwrforoLKGEdYmNgaOyK5fU3oIqCq7N5i_pv_e5hw5mrXTgfRzgfBCDJXIGoTUiGsZJ3cUIgzFl