may someone be so kind to explain how to arrive, step by step, from equation 23 to 28?
Most of all I would like to understand the approximation with delta: if I substitute eq26 in 25 I get a different result (e.g. delta^3 terms).
See the attached image.
Thank you very much.
eq24 may be taken as it is, I mean, phi is simply "(A D Cs / x') - (c D Cs/2)"