According to a law discovered by the 19th-century physician Jean Louis Marie Poiseuille, the velocity (in centimeters/second) of blood r cm from the central axis of an artery is given by the following equation, where k is a constant and R is the radius of the artery. How many centimeters from the central axis of the artery is velocity of blood maximized?
v(r)=k(R^2-r^2)