Some of the highest tides in the world occur in the Bay of  Fundy on the Atlantic Coast of Canada. At Hopewell Cape  the water depth at low tide is about 2.0 m and at high tide  it is about 12.0 m. The natural period of oscillation is  about 12 hours and on June 30, 2009, high tide occurred  at 6:45 Am. Find a function involving the cosine function  that models the water depth D(t) (in meters) as a function  of time t (in hours after midnight) on that day.