A) Let P(t) be written as
P(t) = a cos[b(t - d)] + c
The minimum Pmin and the maximum Pmax of P(t) are given.
Pmin = 4
Pmax = 20
c = (Pmax + Pmin) / 2 = (20 + 4) / 2 = 12
|a| = (Pmax - Pmin) / 2 = (20 - 4) / 2 = 8
We now need to find the number of days t after January 1st at which P(t) is maximum by counting the days of the months from January to May and adding 21 days in June.
t = 31 + 28 + 31 + 30 + 31 + 21 = 172
We now use the period to find b (b > 0) as follows
period = 365 = 2π / b
hence b = 2π / 365
A cosine function without shift has a maximum at t = 0. P(t) has a maximum at t = 172. We can model P(t) by a cos(x) function shifted by 172 to the right as follows:
P(t) = 8 cos[(2π / 365)(t - 172)] + 12
check that P(t) is maximum at t = 172: P(172) = 8 cos[(2π / 365)(172 - 172)] + 12 = 8 cos[(0)] + 12 = 20
b) The graph of P(t) is shown below.