At x=0, the equation for other wave will be −cos(ωt−kx) Because the wave will propagate in the oppositedirection with phase difference of π Negative sign will be used as the boundary is rigid.or y=y1+y2 y=a[cos(kx−ωt)−cos(kx−ωt)] At, x=0,y=0 ∴y2 miust be [−acos(kx+ωt)]