When i have already found beamforming,do i still need to use MRC?i have some problems for them
You are doing transmit beamforming and choosing your vector $\mathbf{f}_A$ so that you beamforming in the "best" direction. So you transmit the signal: $\mathbf{z}=\sqrt{P} \mathbf{f}_A x$, where $\mathbf{z}$ is a length $N_T$ vector. Now you receive the signal: $\mathbf{y} = \sqrt{P}\mathbf{H}\mathbf{f}_A x + \mathbf{n}$, which is a length $N_R$ vector. 

The post-MRC SNR should still be a vector. You will have a SNR for each transmit stream, so $N_T$ SNR values. To compute the SNR of the $k^{\text{th}}$ transmit stream after MRC you take the channel gains from transmit antenna $k$ to each of the $N_R$ receive antennas, $\mathbf{h}_k$, and compute its output power: $\gamma_k = P ||\mathbf{h}_k^H \mathbf{H} \mathbf{f}_A||^2$. Or, you can also equivalently do it all at once by: $||\mathbf{H}^H \mathbf{H} \mathbf{f}_A||^2$.

Your formula is actually computing the pre-MRC SNR, whereas your friends was showing how to get the post-MRC SNR. Hope this helps!