Energy use per capita indexed to 1995 means that per capita energy use for any year is viewed as a percent of per capita use in 1995. Using U.S. Department of Energy data for selected years from 1985 and projected to 2035, the per capita energy use, as a percent of the use in 1995, can be modeled by the function E(t) = 0.000772t3 - 0.0760t2 + 1.83t +87.3 where t is the number of years past 1980.
(a) Find the critical points for this function.
(b) Use the second-derivative test to classify and interpret these critical points.