Circles
Know circumference, area, arcs and sectors.
The rules
- C = 2πr and A = πr².
- Arc length = (angle/360) · 2πr.
- Sector area = (angle/360) · πr².
- An inscribed angle is half its intercepted arc.
Watch out: Mixing up radius and diameter.
Worked example
A circle has radius 9. What is the length of an arc with central angle 40°?
- Arc = (40/360) · 2π · 9.
- (1/9) · 18π = 2π.
Answer: 2π
Try it yourself
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