Circle worksheet - Homeschool Math The radius of a circle is 7.9 cm. Calculate its diameter. 4 b. | | r = 9.3 mm. Calculate the area of the circle. ... 7 b. The diameter of a circle is 6.9 cm. Calculate its area. 8 a. C = 3.9 cm. Calculate the area of the circle. ... Answer Key. Answers are rounded to 2 decimals. 1 a. 18.21 ft: 1 b. 5.26 in: 2 a. 16.33 ft: 2 b. 10.87 ft: 3 a.