Halliday, Resnick & Walker — Waves–II: Problem 88

At a certain point, two waves produce pressure variations given by \(\Delta p_1 = \Delta p_m \sin \omega t\) and \(\Delta p_2 = \Delta p_m \sin(\omega t - \phi)\). At this point, what is the ratio \(\Delta p_r / \Delta p_m\), where \(\Delta p_r\) is the pressure amplitude of the resultant wave, if \(\phi\) is (a) 0, (b) \(\pi/2\), (c) \(\pi/3\), and (d) \(\pi/4\)?