Arc Length and Sector Area (Radians)

Q: Can I use these formulas with angles in degrees?

No, these formulas only work if the central angle is measured in radians. If you are given an angle in degrees, convert it to radians first by multiplying by before substituting into the formulas.

CIE IGCSE Additional Mathematics· 9.1· 15 min read

1. Arc Length Calculation Using Radians★★☆☆☆⏱ 4 min

One radian is the angle subtended at the centre of a circle by an arc whose length is equal to the radius. It follows that an arc of length $s$ subtends an angle of $θ = \frac{s}{r}$ radians at the centre, and rearranging this first principle gives the arc-length formula $s = r θ$ .

The length of an arc of a circle is directly proportional to the central angle $θ$ (in radians) subtended by the arc, and the radius $r$ of the circle. This formula is not provided in your exam, so you must memorise it.

📘 Definition

Arc Length

$s = r θ$

Length of the curved portion of a sector of a circle, where $r$ = radius, $θ$ = central angle in radians.

Example:

For a circle of radius 5 cm with central angle 1.4 rad, arc length $s = 5 \times 1.4 = 7$ cm.

📐 Worked Example

Calculate the arc length of a sector with radius 12 cm and central angle $\frac{3 π}{4}$ radians.

1
Identify the known values: $r = 12$ cm, $θ = \frac{3 π}{4}$ rad
2
$s = r\theta = 12 \times \frac{3\pi}{4}$
3
Simplify the expression: $12 \div 4 = 3$ , so $s = 3 \times 3 π = 9 π$ cm, or 28.3 cm to 3 significant figures if a numerical value is required.

Exam tip:

If the question asks for an exact value, leave your answer in terms of $π$ ; otherwise, use 3 significant figures for numerical answers as per CIE convention.

2. Sector Area Calculation Using Radians★★☆☆☆⏱ 4 min

The area of a sector (the 'pizza slice' shape bounded by two radii and an arc) is calculated using the formula $A = \frac{1}{2} r^{2} θ$ , where $θ$ is the central angle in radians. This formula is also not provided in the exam, so memorisation is critical.

📘 Definition

Sector Area

$A = \frac{1}{2} r^{2} θ$

Total area enclosed by two radii of a circle and the arc between them, with $θ$ in radians.

Example:

For radius 6 m and angle 0.5 rad, area = $0.5 \times 6^{2} \times 0.5 = 9$ m².

📐 Worked Example

A sector has area 48 cm² and radius 8 cm. Find the central angle $θ$ in radians.

1
Rearrange the sector area formula to solve for $θ$
2
$A = \frac{1}{2}r^2\theta \implies \theta = \frac{2A}{r^2}$
3
Substitute the given values: $A = 48$ cm², $r = 8$ cm
4
$\theta = \frac{2 \times 48}{8^2} = \frac{96}{64} = 1.5 \text{ rad}$

3. Area of a Segment★★★☆☆⏱ 4 min

A segment is the region bounded by an arc and a chord of the circle. To find its area, subtract the area of the triangle formed by the two radii and the chord from the area of the sector containing the segment.

📐 Worked Example

Calculate the area of the minor segment of a circle with radius 10 cm and central angle 1.2 radians. Use $sin (1.2) \approx 0.9320$ .

1
First calculate the area of the sector
2
$A_{\text{sector}} = \frac{1}{2}r^2\theta = 0.5 \times 10^2 \times 1.2 = 60 \text{ cm}^2$
3
Next calculate the area of the isosceles triangle formed by the two radii and chord, using $A = \frac{1}{2} ab sin C$
4
$A_{\text{triangle}} = 0.5 \times 10 \times 10 \times \sin(1.2) \approx 50 \times 0.9320 = 46.6 \text{ cm}^2$
5
Subtract the triangle area from the sector area to get segment area
6
$A_{\text{segment}} = 60 - 46.6 = 13.4 \text{ cm}^2 \text{ (3 s.f.)}$

Exam tip:

Always check if the question asks for the minor segment (smaller portion) or major segment (larger portion); for major segments, subtract the minor segment area from the total area of the circle.

4. Compound Shapes with Arcs and Sectors★★★★☆⏱ 5 min

Exam questions often combine arcs, sectors, and other 2D shapes (rectangles, triangles, semicircles) to ask for total perimeter or area. Break these problems down into individual components, calculate each part separately, then add or subtract as required.

📐 Worked Example

A garden is shaped like a rectangle of length 15 m and width 10 m, with a semicircular flower bed attached to one of the 10 m sides. Calculate the total perimeter of the garden, excluding the edge where the flower bed joins the rectangle.

1
First, identify the components of the perimeter: 2 lengths of the rectangle, 1 width of the rectangle, and the curved arc of the semicircle
2
The diameter of the semicircle is 10 m, so its radius $r = 5$ m. The central angle of a semicircle is $π$ radians.
3
Calculate the arc length of the semicircle: $s = r θ = 5 \times π = 5 π$ m
4
Sum the straight edges: $15 + 15 + 10 = 40$ m
5
Total perimeter = $40 + 5 π \approx 55.7$ m (3 s.f.)

Exam tip:

When calculating perimeters of compound shapes, always double-check that you are not including internal edges that are not part of the outer boundary of the shape.

5. Perimeter and Area of Shaded Regions★★★★☆⏱ 6 min

The single most common circular-measure exam question asks for the perimeter and area of a shaded region. These regions are almost always built from sectors and triangles, so the key skill is decomposition: work out which sectors and triangles make up the region, then add or subtract them. To get a perimeter you must also find the straight edges, and this nearly always means calculating a chord or a tangent length first.

📘 Definition

Chord Length

$c = 2 r sin (\frac{θ}{2})$

Length of the chord joining the two ends of an arc, where $r$ is the radius and $θ$ (in radians) is the central angle. It comes from cutting the isosceles triangle formed by the two radii into two right-angled triangles, each with an angle of $\frac{θ}{2}$ at the centre.

Example:

For $r = 10$ cm and $θ = 1.2$ rad, chord $c = 2 \times 10 \times sin (0.6) = 20 \times 0.5646 = 11.3$ cm (3 s.f.).

A segment is the standard shaded region: its area is the sector minus the triangle, $\frac{1}{2} r^{2} θ - \frac{1}{2} r^{2} sin θ$ , and its perimeter is the arc plus the chord, $r θ + 2 r sin (\frac{θ}{2})$ . Other shaded regions combine shapes: the region between two arcs of different radii is a large sector minus a small sector, and a 'kite' bounded by two tangents and two radii has area equal to the kite minus the sector. Whatever the shape, trace its outer boundary carefully so your perimeter includes the right arcs, chords, tangents and radii — and no internal edges.

📐 Worked Example

A chord $A B$ of a circle of radius 6 cm subtends an angle of 2 radians at the centre $O$ . Find (i) the perimeter and (ii) the area of the minor segment cut off by $A B$ .

1
Sketch the minor segment. Its boundary is the minor arc $A B$ plus the chord $A B$ , so its perimeter is arc + chord.
2
Find the arc length using $s = r θ$
3
$s = r\theta = 6 \times 2 = 12 \text{ cm}$
4
Find the chord length using $c = 2 r sin (\frac{θ}{2})$
5
$c = 2 \times 6 \times \sin(1) = 12 \times 0.84147 = 10.098 \text{ cm}$
6
Add the two boundary pieces to get the perimeter
7
$P = 12 + 10.098 = 22.1 \text{ cm (3 s.f.)}$
8
For the area, find the sector area, then subtract the triangle area
9
$A_{\text{sector}} = \frac{1}{2}r^2\theta = \frac{1}{2} \times 6^2 \times 2 = 36 \text{ cm}^2$
10
$A_{\text{triangle}} = \frac{1}{2}r^2\sin\theta = \frac{1}{2} \times 6^2 \times \sin(2) = 18 \times 0.90930 = 16.367 \text{ cm}^2$
11
$A_{\text{segment}} = 36 - 16.367 = 19.6 \text{ cm}^2 \text{ (3 s.f.)}$

📐 Worked Example

A shaded region is bounded by two arcs and two straight edges. The arcs are drawn from the same centre $O$ with radii 4 cm and 10 cm, and each subtends an angle of 1.2 radians at $O$ . The straight edges join the corresponding ends of the two arcs. Find (i) the perimeter and (ii) the area of the shaded region.

1
The region is a large sector (radius 10 cm) with a small sector (radius 4 cm) removed. Its boundary is the outer arc, the inner arc, and two straight edges, each of length $10 - 4 = 6$ cm.
2
Find the outer arc length using $s = r θ$
3
$s_{\text{outer}} = 10 \times 1.2 = 12 \text{ cm}$
4
Find the inner arc length
5
$s_{\text{inner}} = 4 \times 1.2 = 4.8 \text{ cm}$
6
Add both arcs and the two straight edges for the perimeter
7
$P = 12 + 4.8 + 2 \times 6 = 28.8 \text{ cm}$
8
For the area, subtract the small sector from the large sector
9
$A = \frac{1}{2}(10)^2(1.2) - \frac{1}{2}(4)^2(1.2) = 60 - 9.6 = 50.4 \text{ cm}^2$

The chord formula $2 r sin (\frac{θ}{2})$ works when both endpoints lie on the circle, so they are the same distance $r$ from the centre. Sometimes a straight edge instead joins two points that are different distances from the centre — for example a point $C$ on a radius, where $O C \neq = O A$ . Then the isosceles shortcut no longer applies, and you find the edge with the general cosine rule $A C^{2} = O A^{2} + O C^{2} - 2 \cdot O A \cdot O C cos θ$ , where $θ$ is the angle between $O A$ and $O C$ at the centre.

📐 Worked Example

A sector $O A B$ has centre $O$ and radius $O A = O B = 8$ cm, and the angle $A O B = 1.5$ radians. The point $C$ lies on the radius $O B$ with $O C = 3$ cm. The shaded region is bounded by the arc $A B$ , the straight segment $B C$ along $O B$ , and the straight line $C A$ . Find the perimeter of the shaded region.

1
Trace the boundary: the arc $A B$ , then $B C$ back along the radius $O B$ , then the straight line $C A$ . Because $C$ is only 3 cm from $O$ while $A$ is 8 cm from $O$ , the edge $C A$ is neither a radius nor a chord, so it must be found with the cosine rule.
2
Find the arc length with $s = r θ$ :
3
$s_{AB} = 8 \times 1.5 = 12 \text{ cm}$
4
The segment $B C$ lies along the radius $O B$ , so $B C = O B - O C = 8 - 3 = 5$ cm.
5
Apply the cosine rule in triangle $O A C$ , using $O A = 8$ , $O C = 3$ and the included angle $A O C = 1.5$ rad (the same as the angle $A O B$ , since $C$ lies on $O B$ ):
6
$CA^2 = OA^2 + OC^2 - 2 \cdot OA \cdot OC \cdot \cos 1.5 = 8^2 + 3^2 - 2(8)(3)\cos 1.5$
7
$CA^2 = 64 + 9 - 48\cos 1.5 = 73 - 48(0.070737) = 69.605$
8
$CA = \sqrt{69.605} = 8.343 \text{ cm}$
9
Add the three boundary pieces to get the perimeter:
10
$P = s_{AB} + BC + CA = 12 + 5 + 8.343 = 25.3 \text{ cm (3 s.f.)}$

Exam tip:

Before writing any numbers, sketch the region and mark every edge of its boundary. Label each edge as an arc (use $r θ$ ), a chord (use $2 r sin \frac{θ}{2}$ ), a tangent, or a radius, so you know exactly which lengths to calculate and add.

6. Common Pitfalls

Wrong move:

Using degree measures directly in $s = r θ$ or $A = \frac{1}{2} r^{2} θ$

Why:

These formulas are only valid for angles in radians, so using degrees gives a completely incorrect result.

Correct move:

Convert any given degree angle to radians first by multiplying by $\frac{π}{180}$ before substituting into the formulas.

Wrong move:

Forgetting to multiply by $\frac{1}{2}$ in the sector area formula

Why:

The sector area formula is derived from the fraction of the circle's total area, so omitting the ½ halves your answer.

Correct move:

Write the formula out fully every time before substituting values to avoid missing the ½ factor.

Wrong move:

Including internal edges when calculating the perimeter of compound shapes

Why:

Internal edges are not part of the outer boundary of the shape, so adding them overstates the perimeter.

Correct move:

Trace the outer boundary of the shape with your finger as you list each edge to include, to make sure you only count outer edges.

Wrong move:

Calculating the minor segment when the question asks for the major segment

Why:

The major segment is the larger portion of the circle, so using the small central angle gives the wrong area.

Correct move:

If asked for the major segment, either use the reflex angle for $θ$ in the sector area formula, or subtract the minor segment area from the total area of the circle.

Wrong move:

Rounding intermediate values too early in multi-step problems

Why:

Early rounding leads to accumulated error in your final answer, which may fall outside the allowed tolerance range for marks.

Correct move:

Keep at least 4 significant figures for all intermediate calculations, only rounding your final answer to 3 significant figures.

7. Quick Reference Cheatsheet

Formula	Purpose	Valid For	Must Memorise?
$s = r θ$	Calculate arc length	$θ$ in radians	Yes
$A = \frac{1}{2} r^{2} θ$	Calculate sector area	$θ$ in radians	Yes
$A_{segment} = A_{sector} - A_{triangle}$	Calculate segment area	$θ$ in radians	No (derive from memorised formulas)
$A_{triangle} = \frac{1}{2} r^{2} sin θ$	Calculate area of triangle inside a sector	$θ$ in radians or degrees	Yes

8. Frequently Asked

Do I get the arc length and sector area formulas in the exam?

No, CIE IGCSE Additional Mathematics 0606 does not provide these formulas in the exam paper, so you must memorise $s = r θ$ and $A = \frac{1}{2} r^{2} θ$ (both only valid for $θ$ in radians).

Can I use these formulas with angles in degrees?

No, these formulas only work if the central angle $θ$ is measured in radians. If you are given an angle in degrees, convert it to radians first by multiplying by $\frac{π}{180}$ before substituting into the formulas.

Going deeper

syllabusCIE 0606 2025-2027 SyllabusSee unit 9 for circular measure requirements

What's Next

Now that you have mastered arc length and sector area using radians, you are ready to tackle more complex circular measure problems that appear frequently in CIE IGCSE Additional Mathematics 0606 exams. The skills you have learned here will also be foundational if you go on to study A Level Mathematics, where you will use these formulas in calculus applications like related rates and integration. Next, practice applying these formulas to past paper questions to build your speed and accuracy, making sure you avoid the common pitfalls listed in this guide. Pay special attention to compound shape questions, as these are often worth 4-6 marks and are a common differentiator between high-scoring candidates.