Hello,

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.

PS

eq24 may be taken as it is, I mean, phi is simply "(A D Cs / x') - (c D Cs/2)"