The area of a circle increases at a rate of 1 cm- / s. a. How fast is the radius changing when the radius is 6 cm? b. How fast is the radius changing when the circumference is 6 cm? a. Write an equation relating the area of a circle, A, and the radius of the circle, r. (Type an exact answer, using it as needed.) Differentiate both sides of the equation with respect to t. dA dr dt dt (Type an exact answer, using it as needed.) When the radius is 6 cm, the radius is changing at a rate of (Type an exact answer, using it as needed.) b. When the circumference is 6 cm, the radius is changing at a rate of (Type an exact answer, using x as needed.)