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## Circle

 Center $$O$$ Radius $$r$$ Diameter $$d$$ Circumference $$C$$
$$C = 2 \pi r$$
Circumference
$$r = \frac{C}{2 \pi }$$
$$A = \pi {r}^2$$
Area
$$r = \sqrt{\frac{A}{\pi}}$$
$$d = 2 r$$
$$r = \frac{d}{2}$$
3. Pi (symbol $\pi$) constant value approximated as $$\pi \simeq 3.14$$