(a) The volume V of a growing spherical cell is V= , where the radius is measured in micrometers (1 um = 10 5m). Find the average rate of change of V with respect to r when / changes from 3 to each of the following. (Round your answers to one decimal place.) 3 to 6 um um3/um (ii) 3 to 4 um um3/um (iii) 3 to 3.1 um Jum3/um (b) Find the instantaneous rate of change of V with respect to / when r = 3 um. (Round your answer to one decimal place.) V'(3 ) = um / um