Blood flows through an artery of radius R. At a distance r from the central axis of the artery, the speed of the blood flow is given by S(r)=k(Rē - rē). Show that the average speed of the blood flow is one-half the maximum speed.

My problem is that I can't prove that -- I keep proving that it is two-thirds the maximum speed. Can anyone tell me where I am going wrong?

The maxiumum speed is kRē at r = 0 and the function reaches zero when r = R.

Where did I go wrong? I just can't see it. Thanks for the help!