A) \[\frac{\sqrt{3}}{2}\]
B) \[-\frac{1}{2}\]
C) \[\frac{1}{\sqrt{2}}\]
D) \[\frac{1}{2}\]
Correct Answer: B
Solution :
\[\cos \,(-{{840}^{o}})=\cos (-{{36}^{o}}-{{36}^{o}}-{{12}^{o}})\] \[=\cos \,(-{{120}^{o}})\] \[=\cos {{120}^{o}}\] \[=-\frac{1}{2}\]
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