nd $dy/dt$, which gave us
    $Df(x,y) = \begin{bmatrix}\partial f/\partial x&amp;\partial
    f/\partial y\end{bmatrix}$ and $D\vec r(t) =
    \begin{bmatrix}dx/dt\\dy/dt\end{bmatrix}$.</div>
  </div>

  <div class="problem" label="General Chain Rule Formulas">
    <div class="marginpar">
      <span class="bmw">See 14.4: 13-24 for more practice. Practice
      these problems by using matrix multiplication. The examples
      problems in the text use a &ldquo;branch diagram,&rdquo;
      which is just a wa