Abstract-To address the challenges with real-time accurate modeling of multi-segment continuum manipulators in presence of significant external and body loads, we introduce a novel series solution for variable-curvature Cosserat rod static and Lagrangian dynamic method. By combining a modified Lagrange polynomial series solution, based on experimental observations, with Ritz and Ritz-Galerkin methods, the infinite modeling state space of a continuum manipulator is minimized to geometrical position of a handful of physical points (in our case two). As a result, a unified easy to implement vector formalism is proposed for nonlinear impedance and configuration control. We showed that by considering the mechanical effects of axial highly elastic deformation, the model accuracy is increased by up to 6%. The proposed model predicts experimental results with 6-8% (4-6 [mm]) mean error for the Ritz-Galerkin method in static cases and 16-20% (12-14 [mm]) mean error for the Ritz method in dynamic cases, in planar and general 3D motions. Comparing to five different models in the literature, our approximate solution showed to be more accurate with the smallest possible number of modeling states and suitable for real-time modeling, observation and control applications.