Consumer price index an economys consumer price index cpi

Question

Question: CONSUMER PRICE INDEX An economy's consumer price index (CPI) is described by the function

l(t) = -0.2t³ + 3t² + 100          (0 ≤ t ≤ 10)

where t = 0 corresponds to 1998.

a. At what rate was the CPI changing in 2003? In 2005? In 2008?

b. What was the average rate of increase in the CPI over the period from 2003 to 2008?