Following are expressions for four plane simple harmonic waves (i) y1=Acos2π(n1t+λ1x) (ii) y2=Acos2π(n1t+λ1x+π) (iii) y3=Acos2π(n2t+λ2x) (iv) y4=Acos2π(n2t−λ2x) The pairs of waves which will produce destructive interference and stationary waves respectively in a medium, are