Sector Area and Arc Length
Finding areas and lengths of circle sectors
Sector Area and Arc Length
Sector
A sector is a "slice" of a circle, like a piece of pie.
It's bounded by two radii and an arc.
Arc Length
The length of the curved part of the sector.
Formula:
Or in radians:
Sector Area
The area of the "slice."
Formula:
Or in radians:
Segment
The region between a chord and the arc it cuts off.
Area of segment = Area of sector - Area of triangle
Strategy
- Find the fraction of the circle:
- Multiply by the whole circle (circumference or area)
Common Sectors
- Semicircle: → half circle
- Quarter circle: → one-fourth circle
📚 Practice Problems
1Problem 1easy
❓ Question:
A circle has a radius of 6 cm. Find the length of an arc that subtends a central angle of 60°.
💡 Show Solution
Step 1: Understand what we're finding: Arc length is a portion of the circumference
Step 2: Use the arc length formula: Arc length = (θ/360°) × 2πr where θ is the central angle in degrees
Step 3: Substitute values: Arc length = (60°/360°) × 2π(6) Arc length = (1/6) × 12π Arc length = 2π cm
Step 4: Approximate (optional): 2π ≈ 2 × 3.14159 ≈ 6.28 cm
Step 5: Verify the logic: 60° is 1/6 of 360° (full circle) So arc is 1/6 of circumference C = 2π(6) = 12π Arc = 12π/6 = 2π ✓
Answer: Arc length = 2π cm (≈ 6.28 cm)
2Problem 2easy
❓ Question:
Find the arc length of a sector with radius 6 and central angle 60°.
💡 Show Solution
Use the arc length formula:
Answer: (or approximately 6.28) units
3Problem 3easy
❓ Question:
Find the area of a sector with a central angle of 90° in a circle with radius 8 cm.
💡 Show Solution
Step 1: Use the sector area formula: Sector area = (θ/360°) × πr²
Step 2: Substitute values: Sector area = (90°/360°) × π(8)² Sector area = (1/4) × 64π Sector area = 16π cm²
Step 3: Approximate (optional): 16π ≈ 50.27 cm²
Step 4: Verify: 90° is 1/4 of 360° Total circle area = π(8)² = 64π Sector = 64π/4 = 16π ✓
Answer: Sector area = 16π cm² (≈ 50.27 cm²)
4Problem 4medium
❓ Question:
Find the area of a sector with radius 8 and central angle 135°.
💡 Show Solution
Use the sector area formula:
Answer: (or approximately 75.4) square units
5Problem 5medium
❓ Question:
An arc of length 10π cm subtends a central angle of 120° in a circle. Find the radius of the circle.
💡 Show Solution
Step 1: Use arc length formula: Arc length = (θ/360°) × 2πr
Step 2: Substitute known values: 10π = (120°/360°) × 2πr 10π = (1/3) × 2πr 10π = (2π/3)r
Step 3: Solve for r: 10π = (2π/3)r 10π × (3/2π) = r 30π/2π = r r = 15 cm
Step 4: Verify: Arc = (120°/360°) × 2π(15) Arc = (1/3) × 30π Arc = 10π ✓
Answer: The radius is 15 cm
6Problem 6medium
❓ Question:
A sector has an area of 27π square meters and a central angle of 135°. Find the radius of the circle.
💡 Show Solution
Step 1: Use sector area formula: Sector area = (θ/360°) × πr²
Step 2: Substitute known values: 27π = (135°/360°) × πr²
Step 3: Simplify the fraction: 135°/360° = 3/8 27π = (3/8) × πr²
Step 4: Solve for r²: 27π = (3π/8)r² 27π × (8/3π) = r² 216π/3π = r² 72 = r²
Step 5: Solve for r: r = √72 r = √(36 × 2) r = 6√2 meters
Step 6: Verify: Sector area = (3/8) × π(6√2)² = (3/8) × π(72) = (3/8) × 72π = 27π ✓
Answer: The radius is 6√2 meters (≈ 8.49 m)
7Problem 7hard
❓ Question:
A sector has radius 10 and arc length . Find the central angle and the area of the sector.
💡 Show Solution
Step 1: Find the central angle using arc length
Step 2: Find the sector area
Answer: Central angle = , Area = square units
8Problem 8hard
❓ Question:
A pendulum swings through an arc of 18 inches. The pendulum is 24 inches long. Find: (a) the central angle in degrees, and (b) the area of the sector formed.
💡 Show Solution
Step 1: Understand the setup: Arc length = 18 inches Radius (pendulum length) = 24 inches Need to find the central angle θ
Step 2: Use arc length formula to find θ: Arc length = (θ/360°) × 2πr 18 = (θ/360°) × 2π(24) 18 = (θ/360°) × 48π
Step 3: Solve for θ: 18 × 360° = θ × 48π 6480° = 48πθ θ = 6480°/(48π) θ = 135°/π
Step 4: Calculate θ in degrees: θ = 135°/π ≈ 135°/3.14159 ≈ 42.97°
Step 5: Find sector area: Sector area = (θ/360°) × πr²
Using exact value θ = 135°/π: Sector area = (135°/π / 360°) × π(24)² Sector area = (135°/(360°π)) × 576π Sector area = (135° × 576π)/(360°π) Sector area = (135° × 576)/(360°) Sector area = 77760°/360° Sector area = 216 square inches
Step 6: Alternative method for area: Area = (1/2) × arc length × radius Area = (1/2) × 18 × 24 Area = 216 square inches ✓
Answer: (a) Central angle ≈ 42.97° (or exactly 135°/π) (b) Sector area = 216 square inches
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