Following are expressions for four plane simple harmonic waves $$ y_1 = A \cos 2 \pi \left( n_1 t + \frac{x}{\lambda_1} \right), \quad y_2 = A \cos 2 \pi \left( n_1 t + \frac{x}{\lambda_1} + \pi \right), \quad y_3 = A \cos 2 \pi \left( n_2 t + \frac{x}{\lambda_2} \right), \quad y_4 = A \cos 2 \pi \left( n_2 t - \frac{x}{\lambda_2} \right) $$ The pairs of waves which will produce destructive interference and stationary waves respectively in a medium, are