#### Problem 18E

The velocity v of blood that flows in a blood vessel with radius R and length l at a distance r from the central axis is v(r)=P4l(R2r2) where P is the pressure difference between the ends of the vessel and is the viscosity of the blood see Example 2.7.7. Find the average velocity with respect to r over the interval 0rR. Compare the average velocity with the maximum velocity.