Extension of a rod segment dx due to a passing longitudinal wave

[bolzano95]

Let us look at short segment of a rod with its length dx. Due to longitudinal wave, left endpoint moves for s in the direction of x-axis and the right endpoint moves in the same direction for s+ds.

Because I want to calculate the elastic energy of the wave motion, I need the extension of dx so I can apply Hook's law.

But I'm not sure what is my extension: dx-ds or dx+ds? In my textbook its written that the extension is simply ds, but I don't understand why.

 

[Orodruin]

What is the length of the segment in the resting state? How does this change when the ends are displaced as described?