12. DETAILS Manufacturing Cost. Suppose the cost of a new product for the first 10 days is modelled by the equation c(t ) = 4+2 + 2t + 3 where c(t) is the cost in $/day and t is in days. Note: Enter your answers as a fraction or a decimal rounded to four (4) decimal places without units. (a) The average rate of change of the cost between 1 and 9 days is $/day (b) The instantaneous rate of change of the cost when t = 9 is $/day