Radians and circular measure
Use radians to connect an angle directly to lengths and areas.
Pure Mathematics 1 (Paper 1) · Mathematics 9709, 2026–2027. This short revision lesson connects angle units, sector boundaries and segment area; it does not replace the detailed section 1.4 pathway. Written solutions use self-check criteria, not an official mark scheme. Specialist accuracy review has not been completed.
Before you begin
- Use circumference 2πr and area πr².
- Rearrange simple formulae and use triangle area ½ab sin C with consistent angle units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
A circle has radius 8 cm. Which calculation gives the arc length for a central angle of 75°?
What you will learn
- Convert degrees to radians.
- Calculate arc length and sector area.
A radian is a length ratio
An angle of one radian subtends an arc equal in length to the circle's radius. Since a full circumference is 2πr, a full turn is 2π radians. Thus 180° corresponds to π radians.
The formula s=rθ works when θ is in radians. Degrees must be converted first. Keep exact multiples of π until the end if exact form is requested.
Sector area is a fraction of a circle
The sector occupies the fraction θ/(2π) of a full circle. Multiplying that by πr² gives 1/2 r²θ. If asked for a sector perimeter, include both radii as well as the arc.
Sources for this lesson
Pause and explain
A sector has radius 8 cm and angle 1.5 radians. A wire follows its entire boundary. How long is the wire?
Worked example
A sector has radius 9 cm and angle π/3 radians. Find its arc length and perimeter.
Show the worked solution
- Use s=rθ=9×π/3=3π cm.
- The boundary also includes two radii totalling 18 cm.
- Add the straight and curved parts.
Answer Arc length 3π cm; perimeter 18+3π cm
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.
The diagram shows a sector with radius 6 and angle 120°. Taking the lengths in centimetres, derive its exact arc length, area and full perimeter. Explain why the two radii are not part of the arc length.
Give me a hint
Convert 120° to radians, then separate the curved boundary, straight boundary and enclosed area.
Compare my reasoning
- 120° × π/180 = 2π/3 radians. The arc length is 6 × 2π/3 = 4π cm.
- The sector area is ½ × 6² × 2π/3 = 12π cm². This is one third of the full circle area 36π.
- The full perimeter is 4π + 6 + 6 = 4π + 12 cm. The two radii are straight edges, whereas the arc is only the curved edge.
Arc: 4π cm; area: 12π cm²; perimeter: 4π + 12 cm.
Look for these in your work
- Convert the angle before substitution.
- Use squared length units only for area.
- Count two radii for perimeter and none for arc length.
A sector has arc length 15 cm and area 45 cm². Find its radius, central angle in radians and full perimeter. Check your answers in both given measurements.
Give me a hint
Eliminate θ from s = rθ and A = ½r²θ; do not confuse the given arc with the perimeter.
Compare my reasoning
- Since s = rθ, the area is A = ½rs. Thus r = 2A/s = 90/15 = 6 cm.
- The angle is θ = s/r = 15/6 = 2.5 radians. The full perimeter is 15 + 2(6) = 27 cm.
- Check: rθ = 6 × 2.5 = 15 cm and ½r²θ = ½ × 36 × 2.5 = 45 cm². Also 0 < 2.5 < 2π, consistent with a sector smaller than a whole circle.
Radius 6 cm; angle 2.5 radians; perimeter 27 cm.
Look for these in your work
- Eliminate the same angle from both formulae.
- Distinguish the known arc from the full boundary.
- Substitute back into the original length and area.
A circle has radius 5 cm. A chord and its minor arc enclose a segment with central angle 2π/3 radians. Find the exact segment area and perimeter. Explain why subtracting the triangle changes both the area calculation and the straight boundary.
Give me a hint
Sketch two radii and the chord. Remove the isosceles triangle from the sector; use the chord, not two radii, in the segment perimeter.
Compare my reasoning
- The sector area is ½ × 25 × 2π/3 = 25π/3 cm². The triangle area is ½ × 25 × sin(2π/3) = 25√3/4 cm².
- Subtract the triangle: segment area = 25π/3 − 25√3/4 cm². It is positive and smaller than the sector area because 0 < 2π/3 < π.
- Bisecting the isosceles triangle gives chord length 2 × 5 × sin(π/3) = 5√3 cm. The minor arc is 5 × 2π/3 = 10π/3 cm.
- The segment perimeter is 10π/3 + 5√3 cm. The chord replaces the two radii as the straight part of this boundary; it is not added to them.
Area: 25π/3 − 25√3/4 cm²; perimeter: 10π/3 + 5√3 cm.
Look for these in your work
- Identify the minor segment and subtract its central triangle.
- Use the half-angle for the chord and full angle for triangle area.
- Trace only the boundary of the requested region.
Common mistakes
- Using degrees directly in s=rθ gives an incorrect result.
- An arc length is not the entire sector perimeter.
What can you explain now?
Find the area of a sector of radius 4 cm and angle π/2.
Compare with the explanation
4π cm²
Use 1/2×16×π/2=4π.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without notes, explain why s = rθ needs radians. Sketch a sector and its minor segment, and label which straight edges belong to each perimeter. Then derive r = 2A/s for a sector from its area A and arc length s.
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.