Chapter 1.4, Problem 100E

Sales A company that produces snowboards forecasts monthly sales over the next $2$ years to be

$S=23.1+0.442t+4.3cos\frac{\pi t}{6}$

where $S$ is measured in thousands of units and $t$ is the time in months, with $t=1$ corresponding to January 2017. Predict the sales for each of the following months.

$\left(\text{a}\right)$ February 2017

$\left(\text{b}\right)$ February 2018

$\left(\text{c}\right)$ June 2017

$\left(\text{d}\right)$ June 2018