A) ? 2
B) ? 1
C) 1
D) 2
Correct Answer: B
Solution :
\[\cos \,2\alpha +\cos \,2\beta +\cos \,2\gamma \] \[=2\,{{\cos }^{2}}\alpha -1+2\,{{\cos }^{2}}\beta -1+2\,{{\cos }^{2}}\gamma -1\] \[=2\,({{\cos }^{2}}\alpha +{{\cos }^{2}}\beta +{{\cos }^{2}}\gamma )-3=2-3=-1.\]
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