Astronomers believe that the radius of a variable star increases and decreases with the brightness of the star. suppose a variable star has an average radius of 18 million miles and changes by a maximum of 1.5 million miles from this average during a single pulsation, and that the time between periods of maximum brightness is 5.6 days. find an equation that describes the radius of this star as a function of time. (let r be the radius in millions of miles and let t be the time in days. assume that when t = 0 the radius is 18 million miles and increasing.)