A Cepheid variable star is a star whose brightness alternately increases and decreases. For a certain star, the interval between times of maximum brightness is $5\mathrm{.}8$ days. The average brightness of this star is $2\mathrm{.}0$ and its brightness changes by $0\mathrm{.}55.$ Using this information, the brightness of the star at time t$,$ where t is measured in days, has been modeled by the function B$($t$)=2\mathrm{.}0+0\mathrm{.}55$ sin$(2$t$/5\mathrm{.}8)$ Find the rate of change of the brightness at time t days. dB$/$dt$=$

Find the rate of change of brightness at time t $=1$ day. dB$/$dt