## Precalculus (6th Edition) Blitzer

Based on the given data, we can make a graph as shown in the figure. It can be seen that the pattern resembles a cosine curve $y=12+2cos(\omega t)$. As the period is $p=12$ months, we have $\omega=\frac{2\pi}{p}=\frac{\pi}{6}$; thus the function is $y=12+2cos(\frac{\pi}{6} t)$.