#### Answer

(a) C=2r$\pi$
(b) C=D$\pi$
(c) You must divide it by two to find the radius

#### Work Step by Step

(a) You must use this formula if you have the radius because the other formula uses only the diameter
(b) You must use this formula if you have the diameter because the other formula uses only the radius
(c) You have to find the radius from the diameter because the formula for the area of a circle only uses the radius.