gain the parametric surface $\vec r(a,t) = (a\cos t,
    a\sin t, t)$ for $2\leq a\leq 4$ and $0\leq t\leq 4\pi$. We'd
    like to obtain an equation of the tangent plane to this surface
    at the point $\vec r(3,2\pi)$. Once you have a point on the
    plane, and a normal vector to the surface, we can use the
    concepts in chapter 2 to get an equation of the plane. Give an
    equation of the tangent plane. [Hint: To get the point, what is
    $\vec r(3,2\pi)$? The partial derivatives at $(3,2\pi)$ give