Calculate arc lengths, sector areas and sector perimeters
Use radian formulae in both directions, distinguish an arc from the whole boundary and retain consistent length units.
Section 1.4 · Circular measure
Pure Mathematics 1 (Paper 1) · AS & A Level Mathematics 9709, 2026–2027. Use radians in arc and sector formulae. Written practice is self-checked.
Before you begin
- Convert angles to radians.
- Rearrange formulae and distinguish length from area units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
A sector angle is 90°. Which value belongs in s = rθ?
What you will learn
- Use s = rθ and A = r²θ/2 for sectors.
- Recover an unknown radius or angle and calculate a complete sector perimeter.
A sector is a fraction of a circle
A sector subtending θ radians is a fraction θ/(2π) of a complete circle. Multiply this by circumference 2πr to get arc length s = rθ, and by circle area πr² to get sector area A = r²θ/2. These formulae require θ in radians. For an ordinary sector take 0 < θ < 2π and identify whether it is the minor or major sector.
For r = 6 cm and θ = π/3, s = 2π cm and A = 6π cm². The sector boundary contains its curved arc and two radii, so its perimeter is 2π + 12 cm. Area and perimeter answer different questions, and the arc does not include either straight radius. A whole disk has no two-radius boundary, so use its circumference when θ is a complete turn.
Reverse the formula using the quantity you know
If the arc length and radius are known, θ = s/r. If area and radius are known, θ = 2A/r². If the arc length and sector area are known, eliminate θ to obtain A = rs/2, hence r = 2A/s. Substitute back into the original formula and check that the recovered angle fits the intended sector.
For A = 24 cm² and s = 8 cm, r = 2(24)/8 = 6 cm and θ = 8/6 = 4/3 radians. The sector perimeter is 8 + 12 = 20 cm. Convert any mixed units before calculating: a radius in metres and an arc in centimetres cannot be divided as written to get the correct angle.
Pause and explain
A sector has radius 5 and arc length 4. What is its perimeter?
Worked example
A sector has radius 6 cm and central angle π/3. Find its arc length, area and perimeter exactly.
Show the worked solution
- The radian angle can be used directly: s = 6(π/3) = 2π cm.
- The area is A = 6²(π/3)/2 = 6π cm².
- The boundary has two radii and the arc, so its perimeter is 12 + 2π cm.
Answer Arc 2π cm; area 6π cm²; perimeter 12 + 2π cm.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find the arc length, area and perimeter of a sector of radius 9 cm and angle 2π/3.
Give me a hint
Use the radian angle directly, then add the straight sides only for perimeter.
Compare my reasoning
- The arc is 9(2π/3) = 6π cm.
- The area is 9²(2π/3)/2 = 27π cm².
- The perimeter is the arc plus two radii: 6π + 18 cm.
Arc 6π cm; area 27π cm²; perimeter 18 + 6π cm.
Look for these in your work
- Apply both radian formulae with correct powers of r.
- Keep the area and perimeter units distinct.
A sector has area 20 cm² and radius 5 cm. Find its angle, arc length and perimeter.
Give me a hint
Recover θ from the area first, then use s = rθ.
Compare my reasoning
- Rearrange A = r²θ/2 to obtain θ = 2(20)/25 = 8/5 radians.
- The arc length is 5(8/5) = 8 cm.
- Add the two radii to give perimeter 8 + 10 = 18 cm.
Angle 8/5 radians; arc 8 cm; perimeter 18 cm.
Look for these in your work
- Rearrange the area formula before substituting.
- Use the recovered angle consistently for the arc.
A sector-shaped panel has curved edge length 12 cm and area 30 cm². Find its radius, angle and total boundary length.
Give me a hint
Eliminate θ between the arc and area formulae.
Compare my reasoning
- Use A = rs/2, so r = 2(30)/12 = 5 cm.
- Then θ = s/r = 12/5 radians, within the interval for an ordinary sector.
- The total boundary is 12 + 2(5) = 22 cm; check the area 5²(12/5)/2 = 30.
Radius 5 cm; angle 12/5 radians; boundary 22 cm.
Look for these in your work
- Use both supplied quantities to determine r.
- Check the recovered values and include both straight edges.
Common mistakes
- Convert degrees before applying the radian formulae.
- A sector perimeter includes two radii; its arc length does not.
- Use squared length units for area and convert mixed lengths first.
What can you explain now?
A sector has area 24 cm² and arc length 8 cm. Find its radius and angle.
Compare with the explanation
Radius 6 cm; angle 4/3 radians.
Use A = rs/2 to obtain r = 6, then θ = s/r = 4/3.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Derive the arc and sector formulae from the full circle, then explain how you would find r when only A and s are given.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.