The variable star is one whose brightness alternately rises and reduces. For one such star, time between periods of maximum brightness is 4.7 days, average brightness (or magnitude) of star is 5.4, and its brightness differs by ±0.25 magnitude. Determine the function which models brightness of star as function of time (in days), t. (Suppose that at t = 0 brightness of star is 5.4 and that it is increasing.)