How can we install CUDA7.5 on HP-ENVY(interHD+Nvidia950M)+ ubuntu14.04.03 laptop?

I follow the steps on http://docs.nvidia.com/cuda/pdf/CUDA_Installation_Guide_Linux.pdf

I cannot loggin GUI interface of ubuntu14, but ctrl+alt+F2(tty2) can login in.

If you search on this board you will find a number of discussions around login problems.