, then the
        differential is $df=\blank{1cm}\,dx+\blank{1cm}\,dy$ (just
        plug $x=2$ and $y=1$ into the partial derivatives). If we
        move along the surface to $(x,y)=(2.1,1.1)$, then our
        change in $x$ is $\Delta x=\blank{1cm}$, our change in $y$
        is $\Delta y=\blank{1cm}$, and the differential $df$ at
        $(x,y)=(2,1)$ estimates our change in height $\Delta f$ to
        be about $\Delta f\approx \blank{1cm}$ (just plug in
        $\Delta x$ for $dx$ and $\Delta y$ for 