The depth of water, d d\,d metres, at the entrance to a harbour is given by the formula: d=5−4sin(30t)d = 5 - 4\sin(30t)d=5−4sin(30t), where t t\,t is the time in hours after midnight on one day.
a.[4]
On the axes below, draw the graph of d d\,d against t t\,t for 0≤t≤120 \leq t \leq 120≤t≤12
b.[2]
Find the two values of ttt, where 0≤t≤240 \leq t \leq 240≤t≤24, when the depth is least.