 derivative?</li>
    </ol>
  </div>

  <div class="review">
    Suppose $f(x,y)=x^2+3xy$ and $(x,y) = \vec r(t) = (3t,t^2)$.
    Compute both $Df(x,y)$ and $D\vec r(t)$. Then explain how you
    got your answer by writing what you did in terms of partial
    derivatives and regular derivatives.
      <div class="review-answer">Answer: We have
    $Df(x,y) = \begin{bmatrix}2x+3y&amp;3y\end{bmatrix}$ and $D\vec
    r(t) = \begin{bmatrix}3\\2t\end{bmatrix}$. We just computed
    $f_x$ and $f_y$, and $dx/dt$ a