The stationary wave y=2a sin kx cos ωt in a closed organ pipe is the result of the superposition of y=a sin(ω t−kx) and

In closed organ pipe. If  yincident=asin(ωt−kx)then  yreflected=asin(ωt+kx+π)=−asin(ωt+kx)Superimposition of these two waves give the required stationary wave.