Hello myem,
You should get these formulas in the following references...
[1] Mongia, R., Bahl, I. and Bhartia P., RF and Microwave Coupled-Line Circuits, Artech House Inc., Norwood, MA, 1999, pp.370-372
and
[2] B. Bazdar, A.R. Djordjevic, R.F. Harrington, and T.K. Sarkar, "Evaluation of quasi-static matrix parameters for multiconductor transmission lines using Galerkin's method," IEEE Trans. Microwave Theory Tech., vol. MTT-42, July 1994, pp. 1223-1228.
---manju---