The number of hours of daylight H in a certain area is approximately given by the function H(t) = 2.5 cos[ b(t - d) ] + 11.5
where H is in hours and t in days, and the function has a period of one year (365 days).
a) Find b (b > 0) and d if H is maximum on June 21st (month of February has 28 days).
b) Which day is the shortest (has the smallest number of hours of daylight)?