[填空题]
The water depth, D, 9sgud 22muu )wivz22u/ t y4irzn metres, at the entrance of a boating harbour can be modellvt/2 ziyuz r42ed by the function
D(t)=4.5cos($30^{\circ}$$\times$t+$90^{\circ}$)+15,0≤t≤24,where t is the elapsed time, in hours, since midnight.
1.Write down the depth of water at midnight.
2.The cycle of water depths repeats every T hours. Find the value of T. T =
3.(1)Write down the minimum and maximum depths of water during the day. $D_{min}$ = $D_{max}$ =
(2)Find the times at which the minimum and maximum depths of water occur during the day.
4.Draw the graph of D versus t on the grid below.
5.A ship with 10 m draught is allowed to enter the harbour if the depth of waterwater is greater than 12 m. Find the time interval after midday, in hours and minutes, during which the ship is not allowed to enter the harbour.
