\blank{1cm}\end{bmatrix}d\theta.$$
        If we move to $(u,\theta)=(1.9, \pi/2)$, then the change in
        $u$ is $\Delta u=\blank{1cm}$, the change in $\theta$ is
        $\Delta \theta=\blank{1cm}$, and the change in $\vec r$, as
        estimated by the differential $d\vec r$ at
        $(u,\theta)=(2,\pi/3)$, is approximately $\Delta \vec
        r\approx
        \begin{bmatrix}\blank{1cm}\\\blank{1cm}\\\blank{1cm}\end{bmatrix}$.</li>
      </ol>
    </div>

                    
                    