The incident wave in problem 1 [y = 4 sin (3p t - 6p
x)] and the reflected wave
y = -4 sin (3p t + 6p x) produce a standing wave.

(b)(ii) Sketch the standing wave at t = 0s

y = -8 cos3p t sin6p x

[Remember from Q1 that T = 2/3 s]
at t = T/4 = (1/4)(2/3) = 1/6 s
y = -8 cos3p /6 sin6p x
= y = -8 cosp /2 sin6p x
But cos p/2 = 0
Therefore y = 0 everywhere