Given below are some functions of x and t to represent the displacement of an elastic wave.

(a) y = 5 cos (4x ) sin (20t )

(b) y = 4 sin (5x – t/2) + 3 cos (5x – t/2)

(c) y = 10 cos [(252 – 250) πt ] cos [(252+250)πt ]

(d) y = 100 cos (100πt + 0.5x )

State which of these represent

(a) a travelling wave along –x direction

(b) a stationary wave

(c) beats

(d) a travelling wave along +x direction.

Given reasons for your answers.