Determine the circumference. a) its radius is 2.1 cm. b) its diameter is 15 cm

1. Let’s write down the formula for the circumference through the radius: С = 2 * π * R, where R is the radius. Let us find the circumference if R is 2.1 cm, and we take “n” equal to 3.14:

C = 2 * 3.14 * 2.1 = 13.188 (cm).

2. Let us write down the formula for the circumference through the diameter: С = π * D, where D is the diameter. Let’s calculate if D = 15 cm:

C = 3.14 * 15 = 47.1 (cm).

Answer: circumference 13.188 cm; 4.71 cm.