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Application of Cosine Problem

The tide, or depth of the ocean near the shore, changes throughout the day. The water depth $\displaystyle d$ (in feet) of a bay can be modeled by

$\displaystyle d = 35-28\cos{\frac {\pi}{6.2}}{t}$

where $\displaystyle t$ is the time in hours, with $\displaystyle t=0$ corresponding to 12:00 A.M.

(a) Algebraically find the times at which the high and low tides occur.

**How would I do this? Just solve for **$\displaystyle d$ **? Wouldn't I just get one answer? I need two, one for the high and one for the low.**

(b) Algebraically find the time(s) at which the water depth is 3.5 feet.

**Just **$\displaystyle d$ **for 3.5 and solve right? I have one question though...how would you determine **$\displaystyle \cos {\frac {\pi}{6.2}}$ **algebraically?**

I have also attached a picture of the original problem.