Mensuration · JSS 1, SS 2
Length of an arc
Find the length of part of a circle's edge.
- L = θ360° × 2πr
What the letters mean
- θ
- the angle at the centre
- r
- the radius
Worked example
Find the length of an arc with angle 90° in a circle of radius 14 cm (π = 227).
Step 1: Fraction of the circle
90°360° = 14
Step 2: Quarter of the circumference
14 × 2 × 227 × 14 = 14 × 88 = 22 cm
Final answer: 22 cm
See it
Play with this picture from the Angles in a Circle lesson.
∠AOB = 120° = 2 × ∠ACB = 60°