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## Homework Statement

The water depth in a harbour is 21m at high tide and 11m at low tide. One cycle is completed approximately every 12 hrs.

Find an equation.

## Homework Equations

## The Attempt at a Solution

The answer to this problem is y = 5sin 30 (t-3) + 16

A = (M - m) / 2

= (21-11) / 2

= 5

C = (M+m) / 2

= (21+11)/2

=16

Period is 12 hrs.

K value would be 30 because 360 / 12.

But I don't know why it is (t-3) I'm not sure where that came from.