2.
T = 11cos ( pi * t / 12 ) + 71
In this case....t = 2 [ 2 hours after 2 PM ].......so we have
T = 11 cos (pi * 2 /12) + 71 =
11cos (pi/6) + 71 =
11 [ √3/ 2 ] + 71 =
(11/2)√3 + 71 =
[ 11√3 + 142 ] / 2 ≈ 80.53°
Note that this temperature will occur again at -pi/6 because cos (pi/6) = cos (-pi/6)
So......solving this for t...we have
pi * t /12 = -pi / 6
t / 12 = - 1 / 6 multiply both sides by 12
t = -12 / 6 = - 2....i.e., 2 hours before 2 PM = 12 noon
This makes sense.......the temp would increase until 2PM.....and at equal times from 2PM, namely at 12 noon and at 2 PM.....the temps should be equal