$dy$ to get a single
        number).</li>

        <li>If $f(x,y,z)=xy^2+yz^2$, then $$df =
        \blank{1in}\,dx+\blank{1in}\,dy+\blank{1in}\,dz.$$ If we
        are at the input vector $(x,y,z)=(1,-2,3)$, then the
        differential is $df =
        \blank{1cm}\,dx+\blank{1cm}\,dy+\blank{1cm}\,dz$. If we
        move to $(x,y,z)=(0.9, -2.2, 2.8)$, then the change in $x$
        is $\Delta x=\blank{1cm}$, the change in $y$ is $\Delta
        y=\blank{1cm}$, and the change in $z$ is $\Delta
        z=\