Find the circumference with radius 5 using π ≈ 3.14.

Find the circumference with radius 5 using π ≈ 3.14.

• A. 15.7
• B. 25.13
• C. 22.0
• D. 31.4

Answer: (D)

The key idea is that the circumference uses the radius in the formula C = 2πr. Plugging in r = 5 and π ≈ 3.14, you get C ≈ 2 × 3.14 × 5. Multiply 2 by 3.14 to get 6.28, then multiply by 5 to obtain 31.4. So the circumference is about 31.4 units. Common mistakes show up as numbers like 15.7 or 25.13: 15.7 comes from using πr (missing the factor of 2), and 25.13 would come from using 2πr with a smaller radius (like 4) rather than 5. The correct application with r = 5 gives 31.4.

The key idea is that the circumference uses the radius in the formula C = 2πr. Plugging in r = 5 and π ≈ 3.14, you get C ≈ 2 × 3.14 × 5. Multiply 2 by 3.14 to get 6.28, then multiply by 5 to obtain 31.4. So the circumference is about 31.4 units.

Common mistakes show up as numbers like 15.7 or 25.13: 15.7 comes from using πr (missing the factor of 2), and 25.13 would come from using 2πr with a smaller radius (like 4) rather than 5. The correct application with r = 5 gives 31.4.