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.

(a) Find a linear function that models these data, with x equal to the number of years past 1980 and y equal to the energy use per dollar of GDP, as a percent.

(b) Find a cubic function that models these data, with x equal to the number of years past 1980 and y equal to the energy use per dollar of GDP, as a percent.

(c) Graph each model and the data on the same axes. 

(d) Which model indicates that the percent of energy use may increase after 2035?

Energy Use per Dollar of GDP

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