Circles: circumference, area, arcs and sectors

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Learn it Circumference $C = 2\pi r$ and area $A = \pi r^2$. A sector with central angle θ is the fraction $\frac{\theta}{360}$ of the whole circle, for both arc length and area.

Example: Arc length of a 90° sector, radius 8 cm.

Fraction of the circle: 90/360 = 1/4.
$C = 2\pi(8) \approx 50.27$.
Arc ≈ 50.27 ÷ 4 ≈ 12.6 cm.

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