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**1 of 2 killer questions--Trig**

suppose the depth of the tide in a certain harbor can be modeled by y=21-5cos(pi t)/6, where y is the water detph in feet and t is the time in hours. Consider a day in which t=0 represents 12:00 midnight. For that day, when are the first high tide and the first low tide and what is the water depth at each time?

I came up with:

High tide: 7 am, 16 feet; low tide 13 noon, 15 feet.

That is wrong.....can anyone help?