江苏大学2018年601-数学分析
解:添加$z=0$(方向向上)使积分区域成为闭合曲面。
$\Omega :\Sigma +z_{=h+},$
$\begin{align*}\iint_\Sigma (x^2\cos \alpha +y^2\cos \beta +z^2\cos \gamma )ds&=\iint_\Omega (x^2\cos \alpha +y^2\cos \beta +z^2\cos \gamma )ds-\iint_{z_{=h-}}h^2dxdy\\&=\iiint_\Omega \left ( \frac{\partial P}{\partial x}+\frac{\partial Q}{\partial y}+\frac{\partial R}{\partial z}\right )dV-\pi h^3\\&=2\iiint_\Omega (x+y+z)dV-\pi h^3 .
\end{align*} $
变量代换:
$\begin{cases}
x &=r\cos\theta ,0\leq \theta \leq 2\pi, \\
y &=r\sin\theta ,0\leq r\leq h,\\
z &=r\tan\varphi ,0\leq \varphi \leq \frac{\pi}{4} .
\end{cases} $
$|J|=r^2\sec^2\varphi .$
$\begin{align*}\therefore 2\iiint_\Omega (x+y+z)dV-\pi h^3&=2\int_{0}^{\frac{\pi}{4}}d\varphi \int_{0}^{2\pi}d\theta \int_{0}^{h}(r\cos\theta +r\sin\theta +r\tan\varphi )r^2\sec^2\varphi dr-\pi h^3\\&=\frac{1}{2}\pi h^4-\pi h^3.
\end{align*}$