Using pi approximately 3.14, what is the circumference of a circle with radius 5 cm?

Using pi approximately 3.14, what is the circumference of a circle with radius 5 cm?

• A. 31 cm
• B. 31.4 cm
• C. 32 cm
• D. 28 cm

Answer: (B)

The circumference is found using C = 2πr (or C = πd). With a radius of 5 cm, C ≈ 2 × 3.14 × 5 = 31.4 cm. Using the diameter form, d = 10 cm, gives C ≈ πd = 3.14 × 10 = 31.4 cm as well. So the circumference is about 31.4 cm. The other numbers don’t fit this calculation because they result from using different approximations of π or from rounding in a way that doesn’t match 3.14 as the value for π.

The circumference is found using C = 2πr (or C = πd). With a radius of 5 cm, C ≈ 2 × 3.14 × 5 = 31.4 cm. Using the diameter form, d = 10 cm, gives C ≈ πd = 3.14 × 10 = 31.4 cm as well. So the circumference is about 31.4 cm. The other numbers don’t fit this calculation because they result from using different approximations of π or from rounding in a way that doesn’t match 3.14 as the value for π.