Question 57 of 142

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