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: L=θ360°×2πrL = \frac{\theta}{360°} \times 2\pi r

Or in radians: L=rθL = r\theta

Sector Area

The area of the "slice."

Formula: A=θ360°×πr2A = \frac{\theta}{360°} \times \pi r^2

Or in radians: A=12r2θA = \frac{1}{2}r^2\theta

Segment

The region between a chord and the arc it cuts off.

Area of segment = Area of sector - Area of triangle

Strategy

  1. Find the fraction of the circle: θ360°\frac{\theta}{360°}
  2. Multiply by the whole circle (circumference or area)

Common Sectors

  • Semicircle: θ=180°\theta = 180° → half circle
  • Quarter circle: θ=90°\theta = 90° → 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: L=θ360°×2πrL = \frac{\theta}{360°} \times 2\pi r

L=60360×2π(6)L = \frac{60}{360} \times 2\pi(6)

L=16×12πL = \frac{1}{6} \times 12\pi

L=2πL = 2\pi

Answer: 2π2\pi (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: A=θ360°×πr2A = \frac{\theta}{360°} \times \pi r^2

A=135360×π(8)2A = \frac{135}{360} \times \pi(8)^2

A=38×64πA = \frac{3}{8} \times 64\pi

A=24πA = 24\pi

Answer: 24π24\pi (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 5π5\pi. Find the central angle and the area of the sector.

💡 Show Solution

Step 1: Find the central angle using arc length

L=θ360°×2πrL = \frac{\theta}{360°} \times 2\pi r

5π=θ360×2π(10)5\pi = \frac{\theta}{360} \times 2\pi(10)

5π=θ360×20π5\pi = \frac{\theta}{360} \times 20\pi

5=θ360×205 = \frac{\theta}{360} \times 20

5=20θ3605 = \frac{20\theta}{360}

θ=90°\theta = 90°

Step 2: Find the sector area

A=90360×π(10)2A = \frac{90}{360} \times \pi(10)^2

A=14×100π=25πA = \frac{1}{4} \times 100\pi = 25\pi

Answer: Central angle = 90°90°, Area = 25π25\pi 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