Open Journal of Applied Sciences, 2019, 9, 198-214 http://www.scirp.org/journal/ojapps ISSN Online: 2165-3925 ISSN Print: 2165-3917 DOI: 10.4236/ojapps.2019.94017 Apr. 23, 2019 198 Open Journal of Applied Sciences Stability Analysis of Periodic Solutions of Some Duffing’s Equations E. O. Eze, U. E. Obasi, E. U. Agwu Abstract Keywords 1. Introduction ( ) () , x cx gtx pt + + = () pt t ∈