Given below are some functions of x and t to represent the displacement (transverse or longitudinal) of an elastic wave. Some which of these represent (i) a travelling wave, (ii) a stationary wave or (iii) none at all:
(a) `y = 2 cos (3 x) sin (10 t)`
(b) `y = 2 sqrt(x - v t)`
(c ) `y = 3 sin (5 x - 0.5 t) + 4 cos(5 x - 0.5 t)`
(d) `y = cos x sin t + cos 2x sin 2t`

Explanation

(a) Stationary wave (b) Unacceptable function for any wave
(c ) Travelling harmonic wave (d) Superposition of two stationary waves.