圓柱體積
\[ \displaystyle \begin{aligned} &= \text{圓面積} \times \text{長度}\\[6pt] &= \text{圓周率} (\cong 3.1415926) × \text{半徑}^2 \times \text{長度}\\[6pt] &= \pi × \left( \frac{\text{直徑}}{2} \right)^2 \times \text{長度}\\[6pt] &= \frac{\pi}{4} D^2 L \end{aligned} \] 
