U.S. cellular phone subscribership has been reported as shown below for the 1993-2004 period.
a. Fit a linear trend equation to the time series. Using this equation, determine the trend estimate for 2010.
b. Fit a quadratic equation to the time series, then use the equation to determine the trend estimate for 2010.
c. Construct a graph of the time series along with the equations fitted in parts (a) and (b). Which equation appears to better fit the data?