2. Main Idea and New Vocabulary Key Concept: Area of a Circle
Example 1: Find the Area of a Circle
Example 2: Real-World Example
Example 3: Area of Semicircles
Lesson Menu

3. Find the areas of circles.
semicircle
Main Idea/Vocabulary

4. Key Concept

5. Find the Area of a Circle
Find the area of the circle. Use 3.14 for .
Estimate 3 × 4 × 4 = 48
A = r2 Area of a circle
A ≈ 3.14 ● 42 Replace r with 4.
A ≈ 3.14 ● = 4 ● 4 = 16
A ≈ Multiply.
Answer: The area of the circle is approximately square centimeters.
Check for Reasonableness ≈ 48 
Example 1

6. Find the area of a circle with a radius of 20 inches. Use 3.14 for .
A in2
B in2
C. 314 in2
D. 1,256 in2
Example 1 CYP

7. FOUNTAINS Find the area of the fountain shown. Use 3. 14 for 
FOUNTAINS Find the area of the fountain shown. Use 3.14 for . Round to the nearest tenth if necessary.
Estimate 3 × 3 × 3 = 27
A = r2 Area of a circle
A ≈ 3.14 ● Replace r with 2.6.
A ≈ 3.14 ● = 2.6 ● 2.6 = 6.76
A ≈ Multiply.
Example 2

8. Answer: The area is approximately 21.2 square meters.
Check for Reasonableness ≈ 27 
Example 2

9. A. 44 in2
B. 154 in2
C. 176 in2
D. 616 in2
Example 2 CYP

10. Area of Semicircles
Find the area of the semicircle. Use 3.14 for . Round to the nearest tenth if necessary.
Replace r with 4.5
Example 3

11. Area of Semicircles A ≈ 0.5(3.14)(4.5)2 Use 3.14 for .
A ≈ Simplify.
Answer: The area of the semicircle is approximately 31.8 square centimeters.
Example 3

12. Find the area of the semicircle. Use 3. 14 for 
Find the area of the semicircle. Use 3.14 for . Round to the nearest tenth if necessary.
A ft2
B ft2 
C ft2 
D ft2
Example 3 CYP

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