[填空题]
The mean depth, D, in metre(l:sle a.ov)rs, of a mountain lake z0ghxbm dk 6pyj-x2,, fluctuates in a yearly cycle and caxh yx,-j0pk bd,z26m gn be modelled by the function
D(t)=acos(kt)+b,where t is the elapsed time, in months, since the beginning of an autumn season.
The mean depth of the lake on month 11 is 33.2 m and on month 5 is 22.8 m.
1.Find the value of k, in degrees. $^{\circ}$
2.Set up a pair of equations and find the value of a and the value of b. Give your answers correct to the nearest integer. a ≈ b ≈
3.Hence find the mean depth of the lake on month 8. m
4.Draw the graph of y=D(t) on the grid below, for one full year, indicating clearly the minimum and maximum points.
