回顾：圆的相似性

Here's a wonderful video detailing the definition of $\pi$ as well as its historical origins!

$\pi$ appears in numerous circle formulas.

Notable examples include:

• circumference = $\pi \times$diameter
• area$_{circle}$ = $\pi \times$radius$^2$
• area$_{ellipse}$ = $\pi \times$radius$_1\times$radius$_2$
• volume$_{sphere}$ = $\frac{4}{3}\pi\times$radius$^3$