
$v=\omega \cos (\omega t-k x+\phi)$
at $t=0, x=0, y=-0.5, v>0 \Rightarrow \phi=-\frac{\pi}{6}$
$\therefore y=\sin \left(\omega t-k x-\frac{\pi}{6}\right)$
$(a)$ $\left(x^2-v t\right)^2$
$(b)$ $\log \left[\frac{(x+v t)}{x_0}\right]$
$(c)$ $e^{\left\{-\frac{(x+v t)}{x_0}\right\}^2}$
$(d)$ $\frac{1}{x+v t}$
$\left(t_{0}\right.$ represents the instant when the distance between the source and observer is minimum)
Statement $-2$ : Due to the motion of source, wavelength of the sound waves (emitted by source) as received by stationary listener is affected.
Statement $-3$ : If receiver and source both are moving, the observed frequency must be different from the original frequency of source.
Treat motion of source or listener as always along a line joining them for all above cases.