trig
posted by Kelslie on .
A city averages 14 hours of daylight in June, 10 in December, 12 in both March and September. Assume that the number of hours of daylight varies sinusoidally over a period of one year. Write an expression for n, the number of hours of daylight, as a cosine function of t. Let t be in months and t=0 correspond to the month of January.
My work:
y=2cos((pi/6(xd))+12
Ok how would I get the phase shift? Thanks in advance

treat one of the data values as input into your partial equation.
e.g. 14 hours in June  y = 14, x = 5
14 = 2cos((pi/6(5d))+12
2 = 2cos((pi/6(5d))
1 = cos((pi/6(5d))
so pi/6(5d) = 2pi , because cos 2pi = 1
which solves for d = 7
your equation is
y=2cos(pi/6)(x+7) + 12
test it for one other given value.
e.g. Dec > x = 11
y = 2cos(pi/6)18 + 12
= 2cos(3pi) + 12
= 2(1) + 12
= 10 , as given 
dad