Modeling Tides: When is the First High Tide and Low Tide?

[TonyC]

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?

 

[VietDao29]

\cos 0 = 1
\cos \pi = -1
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EDIT:
Do not double post.
Viet Dao,

 

[LENIN]

It's really qouit simpel you acctuaelly want to know when cos(pi*t/6) equals 1 and -1 (max and min). After you have done this you only have to find y for the calculated values of t. The answeres I got this way where: high tide at 6AM and 6PM with 26 feet and low tide at midnigth and noon with 16 feet.

 

[TonyC]

The light is shining...thank you