COS 2(\alpha +\BETA )=2{{\cos }^{2}}(\alpha +\beta )-1,\,2{{\SIN }^{2}}\beta =1-\cos 2\beta \] L.H.S. \[=-\cos 2\beta +2\cos (\alpha +\beta )\,[2\sin \alpha \sin \beta +\cos (\alpha +\beta )]\] \[=-\cos 2\beta +2\cos (\alpha +\beta )\cos (\alpha -\beta )\] \[=-\cos 2\beta +(\cos 2\alpha +\cos 2\beta )=\cos 2\alpha \].