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Multiple-input multiple-output (MIMO) systems assisted by multiple relays with single antenna are considered. The signal transmission consists of two phases. In the first phase, the source node broadcasts the vector symbols to all relays, then all relays forward the symbols multiplied by gain parameters to the destination simultaneously. Unlike the case of full cooperation among relays such as a single relay with multiple antennas, in this case there may be no closed form optimal solution for the relay gains with respect to utility and/or cost functions due to the distributed relays. Therefore we propose an alternative approach in which we use the relay selection scheme along with conventional methods to find the relay gains. As a utility or cost function, we choose the determinant and trace of mean square error (MSE) matrix which are related to capacity and MMSE respectively. Further we combine this approach with a greedy search algorithm in order to reduce computational complexity. We provide simulation results to validate that the proposed methods incur a negligible performance loss compared to the optimal solution.