# Trig functions

The average depth of the water in a port on a tidal river is 4m. At low tide, the depth of the water is 2m. One cycle is completed approximately every 12h.

a)Find an equation of the depth, d(t)metres, with respect to the average depth, as a function of the time, t hours, after low tide, which occurred at 15:00.

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1. cos(kt) has period 2pi/k
cos(t) has max at t=0, so -cos(t) has min at t=0

y = 4 - 2cos(pi/6 (t-15))

http://www.wolframalpha.com/input/?i=4+-+2cos%28pi%2F6+%28t-15%29%29+

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