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**needhelpplease** The depth, D(t) metres, of water at the entrance to a harbour at t hours after midnight on a

particular day is given by D(t) = 10 + 3 sin(π t/6), 0 ≤ t ≤ 24.

a) Find the value of t for which D(t) ≥ 8.5.

b) Boats which need a depth of w metres are permitted to enter the harbour only if the

depth of the water at the entrance is at least w metres for a continuous period of 1 hour.

Find, correct to 1 decimal place, the largest value of w which satisfies this condition.

i was able to sketch the graph, just having trouble with how you figure out D(t) ≥ 8.5

the answer in the book is {t : D(t) ≥ 8.5} = {t : 0 ≤ t ≤ 7} ∪

{t : 11 ≤ t ≤ 19} ∪ {t : 23 ≤ t ≤ 24}

this makes sense, i just dont know how to figure it out algebraically