Photovoltaic solar power has become more affordable in recent years. Solar power plants project that the average cost per watt of electricity generated by solar panels will decrease exponentially as shown in the graph below:
Assuming that the cost per watt of electricity generated by solar panels will decrease according to the exponential decay model.
a) using the data for 2009 and 2013, find the value of k, and write an exponential function that describes the cost per watt of electricity after time t, in years, where t is the number of years since 2009.
b) Estimate the cost per watt of electricity in 2015 and in 2018.
c) At this decay rate, in which year will the average cost per watt be $1.85?