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Modeling with sinusoidal functions

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Vlad is playing on a swing set.
His horizontal distance D(t) (in m) from the center (where being behind the center means a negative distance) as a function of time t (in seconds) can be modeled by a sinusoidal expression of the form acos(bt)+d.
At t=0, when he pushes off, he is 1 m behind the center, which is as far back as he goes. The swing reaches the center π6 seconds later.
Find D(t).
t should be in radians.