Blood flow resistance. The equation S ( r 4 can be used to determine the resistance to blood flow, S, of a blood vessel that has radius r, in millimeters (mm) a) Find the rate of change of resistance with respect to r, the radius of the blood vessel. b) Find the resistance at /=1.2 mm. c) Find the rate of change of S with respect to r when r = 0.8 mm. d) Interpret and explain the meaning of your answer in part (c)