Acepheid variable star is a star whose brightness alternately increases and decreases. suppose that cephei joe is a star for which the interval between times of maximum brightness is 5.1 days. its average brightness is 3.6 and the brightness changes by +/-0.55. using this data, we can construct a mathematical model for the brightness of cephei joe at time t, where t is measured in days:
b(t)=3.6+0.55sin(2pi(t)/5.1)

i found the rate of change.. it's
0.677598cos(1.232t)

but, i also need to find the rate of increase after one day. (correct to two decimal places)

can someone ?