[填空题]
The mean depth, D, in metres, of a mountain lake fluc7lx egc84hsc qo tn7bcp3).f 7tuates in a yearly cyx.houqrcq)zvz)12 sdoe )b: +cle and can be modebxqez.)vdh2)+orq s c1z) o:ulled 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 1 is 33.2 m and on month 5 is 22.8 m.
1.Find the value of k, in degrees. k = $^{\circ}$
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. D =
4.Draw the graph of y=D(t) on the grid below, for one full year, indicating clearly the minimum and maximum points.
