Solution (source code)

= Solution

Eliminating $T$ from the two boundary conditions gives
$$
\frac{1+R}{1-R}
=\frac{\lambda'k'\cos\theta}
{\lambda k\cos\theta'}.
$$
The <Snell law for elastic and acoustic waves> implies
$$
\frac{k'}k=\frac{\sin\theta}{\sin\theta'}.
$$
Therefore the <displacement reflection from an interface between two inviscid elastic liquids> is
$$
\boxed{
R=\frac{\lambda'\sin2\theta-\lambda\sin2\theta'}
{\lambda'\sin2\theta+\lambda\sin2\theta'}}.
$$