Chapter 16, Problem 28RE

To determine

To calculate: The rate at which revenue to the nearest dollar be changing $10$ months from now for the sales department of OHaganBooks.com which predicts that the revenue from sales of the latest blockbuster Elvish for Dummies will vary in accordance with annual releases of episodes of the movie series “Lord of the Rings Episodes $9-12$ ”. It has come up with the model $R\left(t\right)=20,000+15,000e^{-0.12t}\cos \left[\frac{\pi }{6}\left(t-4\right)\right]$ dollars $0\le t\le 72$, where t is the time in months from now and $R\left(t\right)$ is the monthly revenue.