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In cognitive radio (CR) networks, the secondary users (SUs) try to communicate opportunistically in the frequency band originally allocated to a primary network. This communication should be in a manner that the quality of service (QoS) is satisfied for both primary and secondary networks. In this paper, the considered system consists of a primary broadcasting network with one transmitter and M receivers, and a relay-assisted secondary network with one transmitter and one receiver. In this scheme, if the direct link between the cognitive transmitter and receiver is not maintained the target signal to interference plus noise ratio (SINR), the relaying process, begins where N relays assist the transmitter to transmit its data to the receiver. This relaying process continues until the target SINR is met at the cognitive receiver. This scheme employs joint power allocation and beamforming (BF) at the relay nodes and defines two optimization problems in a way that the objective in the first problem is minimization of the total transmission power of the relays. If there is not any feasible solution for the first problem, the second problem is proposed with the objective of maximizing the receive rate at the destination node. Genetic algorithm (GA) is applied to both defined problems to find optimum transmission power values and beamforming weight in the relay nodes. Simulation results show that applying beamforming scheme results in a considerable reduction in the total transmission power of the relay nodes, while satisfying the QoS in both primary and secondary networks.