blank{1cm}$. The differential $df$ at $(x,y,z)=(1,-2,3)$
        helps us estimate the change in $f$ to be about $\Delta
        f\approx \blank{1cm}$. [Hint: plug in our numeric $x$, $y$,
        $z$, $\Delta x$, $\Delta y$, and $\Delta z$.]</li>

        <li>When a function has multiple outputs, we view the
        differential as a multiple-row and 1-column matrix, where
        each row has the partial derivative of one of the outputs.
        If $\vec r(t)=(3t^2, 2/t)$, then $$d\vec r =
        \begin{