Circumference of a Circle: $C = \pi d$ or $C = 2\pi r$, where $d$ is the diameter and $r$ is the radius.

Area of a Circle: $A = \pi r^2$, where $r$ is the radius. This gives the total space inside the circle.

Arc Length and Area of a Circle Sector: For a sector with central angle $\theta$, the arc length is $(\theta / 360) \times 2\pi r$, and the area of a sector is $(\theta / 360) \times \pi r^2$.

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