28. The problem asks for “the greatest value of x… exactly out of phase” which is to be interpreted as the value of xwhere the curve shown in the figure passes through a phase value of πradians. This happens as some point Pon the xaxis, which is, of course, a distance xfrom the top source and (using Pythagoras’ theorem) a distance d2+ x2 from the bottom source. The difference (in normal length units) is therefore
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