u have a
      point and a vector, we know (from chapter 2) how to get an
      equation of a line.]</li>
    </ol>
  </div>In the previous problem, you should have noticed that the
  partial derivatives of $\vec r(a,t)$ are tangent vectors to the
  surface. Because we have two tangent vectors to the surface, we
  should be able to use them to construct a normal vector to the
  surface, and from that, a tangent plane. That's just cool.

  <div class="review">
    If you know that a plane passes through the 