Diagram
Definition
Core Concepts
- Total angle around a point = \(360^\circ\)
- Sector area is directly proportional to central angle
- Arc length is also proportional to central angle
- Arc length is also proportional to central angle
- Major sector = Whole circle − Minor sector
Important Formula
Derivation of Formula
Since a full circle has area \( \pi r^2 \) corresponding to \(360^\circ\), the sector with angle \( \theta \) will have proportional area:
\[ \frac{\theta}{360} = \frac{\text{Area of Sector}}{\pi r^2} \]Rearranging gives:
\[ \text{Area of Sector} = \frac{\theta}{360} \times \pi r^2 \]Similar proportional reasoning gives arc length formula.
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Common Mistakes
CBSE Case Study / HOTS
This type of question tests multi-step application and appears frequently in board exams.
Why This Topic Matters for Board Exams
Consider a chord AB subtending angle \( \theta \) at the centre O.
Area of sector OAB:
\[ \frac{\theta}{360} \times \pi r^2 \]Area of triangle OAB:
\[ \frac{1}{2} r^2 \sin\theta \]Therefore, minor segment area:
\[ \text{Segment Area} = \text{Sector} - \text{Triangle} \]- Area of Sector
- \[ar_{\text{Sector}} = \frac{60}{360} \times \pi \times 14^2 = \frac{196\pi}{6}\]
- Area of Triangle
- \[\begin{aligned}ar_{\text{Triangle}} &= \frac{1}{2} \times 14^2 \times \sin 60^\circ \\&= 98 \times \frac{\sqrt{3}}{2}\\&=49\sqrt{3}\end{aligned}\]
- Area of Segment
- \[ar_{\text{Segment}} = ar_{\text{Sector}} - ar_{\text{Triangle}}\]
- \[ar_{\text{Segment}}= \frac{196\pi}{6} - 49\sqrt{3}\]
CBSE Case Study / HOTS
Question: A chord subtends \(120^\circ\) at the centre. Find the ratio of segment area to sector area.
Concept: Analytical comparison using formulas.
This tests understanding beyond formula application and is frequently asked in case-based questions.
Area of Minor and Major Sector (30°)
- \(\text{Area of sector} = \frac{\theta}{360} \times \pi r^2\)
- Major sector = Whole circle − Minor sector
- Find area of minor sector (30°)
- Find area of full circle
- Subtract to get major sector
- Given: \( r = 4 \, cm,\ \theta = 30^\circ \)
- Area of Minor Sector
- \[\begin{aligned}\text{Area} &= \frac{30}{360} \times 3.14 \times 4^2\\ &= \frac{1}{12} \times 3.14 \times 16\\ &= \frac{50.24}{12} \approx 4.19 \; \mathrm{cm^2}\end{aligned}\]
- Area of Whole Circle
- \[= 3.14 \times 16 = 50.24 \; \mathrm{cm^2}\]
- Area of Major Sector
- \[= 50.24 - 4.19 = 46.05 \; \mathrm{cm^2}\]
Minor sector ≈ 4.19 cm²
Major sector ≈ 46.05 cm²
- Segment = Sector − Triangle
- Area of sector = \( \frac{\theta}{360} \pi r^2 \)
- Area of triangle = \( \frac{1}{2} r^2 \sin \theta \)
- Find area of sector (120°)
- Find area of triangle OAB using sine formula
- Subtract to get segment area
- Given: \( r = 21 \, cm,\ \theta = 120^\circ \)
- Area of Sector
- \[ \begin{aligned} A_s &= \frac{120}{360} \times \frac{22}{7} \times 21^2\\ &= \frac{1}{3} \times \frac{22}{7} \times 441\\ &= \frac{1}{3} \times 22 \times 63 = 462 \; \mathrm{cm^2} \end{aligned} \]
- Area of Triangle OAB
- \[ \begin{aligned} A_t &= \frac{1}{2} r^2 \sin 120^\circ\\ &= \frac{1}{2} \times 441 \times \frac{\sqrt{3}}{2}\\ &= \frac{441\sqrt{3}}{4} \end{aligned} \]
- Area of Segment
- \[ \begin{aligned} A_{segment} &= 462 - \frac{441\sqrt{3}}{4}\\ &= \frac{1848 - 441\sqrt{3}}{4} \end{aligned} \]
Final Answer: \( \frac{1848 - 441\sqrt{3}}{4} \, cm^2 \)
Chapter Overview
Key Concepts at a Glance
A circle is the locus of all points equidistant from a fixed centre. The constant distance is called the radius (r). The longest chord through the centre is the diameter (d = 2r). In this chapter, we use π ≈ 22/7 or 3.14 unless stated.
A sector is the region between two radii and the arc they cut. The central angle θ° determines what fraction of the circle is used.
The minor segment is the smaller region; the major segment is the larger. The key idea: subtract the triangle from the sector to get the segment.
= (θ/360)πr² − (r²/2)sin θ
Complete Formula Sheet
| Quantity | Formula | Where π = 22/7 or 3.14 |
|---|---|---|
| Circumference of Circle | 2πr | r = radius |
| Diameter | d = 2r | — |
| Area of Circle | πr² | r = radius |
| Area of Semi-circle | πr² / 2 | — |
| Perimeter of Semi-circle | πr + 2r | diameter + arc |
| Length of Arc (sector) | (θ/360) × 2πr | θ = central angle (°) |
| Area of Sector | (θ/360) × πr² | — |
| Perimeter of Sector | l + 2r | l = arc length |
| Area of Minor Segment | (θ/360)πr² − (r²/2)sinθ | θ = central angle |
| Area of Major Segment | πr² − (minor segment area) | — |
| Area of equilateral △ (side a) | (√3/4)a² | — |
| Circumradius of equilateral △ | r = a/√3 | a = side |
| Inradius of equilateral △ | r = a/(2√3) | — |
Special Angle Reference
| θ | sin θ | cos θ | tan θ | Triangle Area = ½r²sinθ (for r=7) |
|---|---|---|---|---|
| 30° | 1/2 | √3/2 | 1/√3 | 12.25 |
| 45° | 1/√2 | 1/√2 | 1 | 17.32 |
| 60° | √3/2 | 1/2 | √3 | 21.22 |
| 90° | 1 | 0 | ∞ | 24.5 |
| 120° | √3/2 | -1/2 | -√3 | 21.22 |
| 150° | 1/2 | -√3/2 | -1/√3 | 12.25 |
| 180° | 0 | -1 | 0 | 0 |
Area of Sector — Radian Form
If the angle is in radians (rarely asked at Class X but useful to know):
θ in radians
θ in radians
Step-by-Step AI Solver
to see the full solution here.
Concept-Building Questions
Concept 1 — Circumference & Area of Circle
🔵 Circumference & Area
🍕 Sector & Arc
🌙 Segment
🔷 Combined Figures
Tips, Tricks & Shortcuts
22/7 × 14 = 44
22/7 × 21 = 66
22/7 × 28 = 88
22/7 × 35 = 110
22/7 × 49 = 154
22/7 × 196 = 616
22/7 × 441 = 1386
r = 14 → A = 616 cm²
r = 21 → A = 1386 cm²
r = 10 → A = 314 cm² (π=3.14)
r = 5 → A = 78.5 cm²
r = 3.5→ A = 38.5 cm²
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