Textbook Problem

A company that produces snowboards forecasts monthly sales for 1 year to be

$S=53.5+40.5\cos \frac{\pi t}{6}$ where S is the sales (in thousands of dollars) and t is the time (in months), with $t=1$ corresponding to January.

(a) Use a graphing utility to graph S.

(b) Use the graph to determine the months of maximum and minimum sales.

(c) Using the graph, will the sales be below $20 thousand during the year? If so, during which months?