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) = p 4l (r2 − r2) where p is the pressure difference between the ends of the vessel and is the viscosity of the blood (see example. ). Find the average velocity (with respect to r) over the interval 0 ≤ r ≤ r. (use eta for . Use capital p for pressure. ).