# Circumference and area of circles

> Grade 8: circumference = pi x diameter, area = pi x radius squared, pi is about 3.14, radius from diameter, area of a semicircle.

Source: https://olyclass.com/library/math-g8-circles-area-en/

- Subject: Math
- Level: Grade 8
- Practice: 16
- Play it free on one screen (projector, TV or whiteboard): https://olyclass.com/library?play=math-g8-circles-area-en
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## Questions

1. Which formula gives the circumference of a circle with diameter d? (2 × d / π × d / π × d ÷ 2 / π × d²)
   Answer: π × d
2. A circle has diameter 10 cm. Using π = 3.14, what is its circumference? (15.7 cm / 31.4 cm / 314 cm / 62.8 cm)
   Answer: 31.4 cm
3. A round pond has diameter 5 m. Using π = 3.14, what is its circumference? (15.7 m / 31.4 m / 7.85 m / 78.5 m)
   Answer: 15.7 m
4. Which formula gives the area of a circle with radius r? (2 × π × r / π × d / π × r × 2 / π × r²)
   Answer: π × r²
5. A circle has radius 3 cm. Using π = 3.14, what is its area? (18.84 cm² / 28.26 cm² / 84.78 cm² / 9.42 cm²)
   Answer: 28.26 cm²
6. A circle has radius 10 cm. Using π = 3.14, what is its area? (31.4 cm² / 314 cm² / 3140 cm² / 62.8 cm²)
   Answer: 314 cm²
7. A circular plate has diameter 20 cm. What is its radius? (10 cm / 20 cm / 40 cm / 5 cm)
   Answer: 10 cm
8. A circle has diameter 8 cm. Using π = 3.14, what is its area? (100.48 cm² / 200.96 cm² / 25.12 cm² / 50.24 cm²)
   Answer: 50.24 cm²
9. A circle has area 314 cm² (π = 3.14). What is its radius? (10 cm / 100 cm / 31.4 cm / 5 cm)
   Answer: 10 cm
10. A semicircle is half of a circle with radius 10 cm. Using π = 3.14, what is its area? (157 cm² / 314 cm² / 62.8 cm² / 78.5 cm²)
   Answer: 157 cm²
11. The value 3.14 is only an approximation of pi.
   Answer: True
12. If you double the radius of a circle, its area also doubles.
   Answer: False
13. The area of a semicircle is half the area of the full circle with the same radius.
   Answer: True
14. A circle has circumference 31.4 cm. Using π = 3.14, what is its diameter in cm?
   Answer: 10 cm
15. What is the radius of a circle with diameter 14 cm?
   Answer: 7 cm
16. A semicircle has radius 2 cm. Using π = 3.14, what is its area in cm²?
   Answer: 6.28 cm²