The Zisman equation, the Fowkes equation and the Owens-Wendt equation, which are derived based on the Berthelot geometric averaging rule for the adhesion work of solid-liquid interface, are theoretical foundations of experimental measurements of surface tensions of solid-liquid interfaces. We generalize the Berthelot geometric averaging rule and obtain a generalized expression of the parameters in Zisman equation. We also obtain a generalized Fowkes equation and a generalized Owens-Wendt equation.