You haven't posted the entire question, so it's impossible to explain why the values of x1, x2 and x3 are as shown. But I assume that you have been given a wavelength that originates from S1 and S2 and are looking for positions along the x axis where the waves are in phase. Consequently the length of the hypotenuse must be equal to the length of the base plus a whole number of wavelengths, so that the waves are in phase at P. Using Pythagoras:
(x+nL)^2 = 4^2 + x^2
where L is wavelength. Set n = 1 and solve, then n=2, and finally n=3. I think you'll find that the answers are inverted however - I believe x1=7.5m, x2=3m, and x3=1.2m.
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Last edited by ChipB; Sep 10th 2014 at 08:42 AM.
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