The present work is devoted to using an analytic approach, namely the homotopy analysis
method, to obtain convergent series solutions of strongly nonlinear problems. On the basis
of the homotopy derivative concept described in Liao (2009) [3], a theorem is proved
here which generalizes some lemmas and theorems provided in Liao (2009) [3] and
Molabahrami and Khani (2007) [4]. Significant applicability of the theorem obtained here
in some practical situations is demonstrated.