Halliday, Resnick & Walker — Waves–II: Problem 96

Four sound waves are to be sent through the same tube of air, in the same direction: \[s_1(x, t) = (9.00 \text{ nm}) \cos(2\pi x - 700\pi t)\] \[s_2(x, t) = (9.00 \text{ nm}) \cos(2\pi x - 700\pi t + 0.7\pi)\] \[s_3(x, t) = (9.00 \text{ nm}) \cos(2\pi x - 700\pi t + \pi)\] \[s_4(x, t) = (9.00 \text{ nm}) \cos(2\pi x - 700\pi t + 1.7\pi)\] What is the amplitude of the resultant wave? (Hint: Use a phasor diagram to simplify the problem.)