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As a promising solution to increase usages of the underutilized spectrum, Cognitive Radio Network (CRN) systems face many new practical challenges. Spectrum sensing and associated controlling algorithms are crucial tools toward dynamic spectrum management. Exploring the spatial diversity, distributed cooperative spectrum sensing can effectively overcome the hidden primary user issue. The performance of the distributed sensing scheme is not only affected by wireless channels but also by the number of CRs. However, CRs join and leave CRN at will and thus the performance of cooperative sensing is varying accordingly. In this paper, we study the impact of CR dynamic activities on cooperative spectrum sensing. Closed form expressions for the dynamic performance of distributed energy-based cooperative spectrum sensing are derived. These closed form expressions are analytical tractable and provide a feasible framework to apply optimization techniques for system design parameters to efficiently guarantee the global optimality without computationally costly exhaustive search. This efficient real-time performance evaluation capability is critical for CRN to perform quick online adaptation, especially because CRN is operated in dynamic scenarios which consist of the wireless links, the primary users' activities, and CR dynamics.