Energy use per dollar of GDP indexed to 1980 means that energy use for any year is viewed as a percent of the use per dollar of GDP in 1980. The following data show the energy use per dollar of GDP, as a percent, for selected years from 1985 and projected to 2035.

These data can be modeled with the function
E(t) = 0.0039(0.4t + 2)3 - 0.13(0.4t + 2)2 - 1.4(0.4t + 2) + 91
where E(t) is the energy use per dollar of GDP (indexed to 1980) and t is the number of years past 1980.
(a) Use this model to find and interpret the instantaneous rates of change of energy use per dollar of GDP in 2000 and 2025.
(b) Use the data in the table to find an average rate of change that approximates the 2025 instantaneous rate.

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Energy Use per Dollar of GDP Year Percent Year Percent 1985 83 2015 51 1990 79 2020 45 1995 75 2025 41 2000 67 2030 37 2005 60 2035 34 2010 56