Question

Evaluate the surface integral. ??s (x2 y2 z2 ds s is the part of the cylinder x2 y2 = 4 that lies between the planes z = 0 and z = 2, together with its top and bottom disks.

Answer 1

The surface $S$ can be parameterized by

$\mathbf r(\theta,z)=(2\cos\theta,2\sin\theta,z)$

where $0\le\theta\le2\pi$ and $0\le z\le2$. Then the surface integral can be computed with

$\displaystyle\iint_S(x^2+y^2+z^2)\,\mathrm dS$
$\displaystyle=\int_{\theta=0}^{\theta=2\pi}\int_{z=0}^{z=2}(4\cos^2\theta+4\sin^2\theta+z^2)\|\mathbf r_\theta\times\mathbf r_z\|\,\mathrm dz\,\mathrm d\theta=\dfrac{128\pi}3$