code class$=$"asciimath"$>$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$((2\backslash$pi t$)/(4\mathrm{.}1))($a$)$ Find the rate of change of the brightness at time t days. $($dB$)/($dt$)=($b$)$ Find the rate of change of brightness at time t$=5$ days. $($dB$)/($dt$)=$