Trigonometric-Identities Question 384
Question: $ \cos A+\cos (240{}^\circ +A)+\cos (240{}^\circ -A)= $
[MP PET 1991]
Options:
A) $ \cos A $
B) 0
C) $ \sqrt{3}\sin A $
D) $ \sqrt{3}\cos A $
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Answer:
Correct Answer: B
Solution:
$ \cos A+\cos ,(240^{o}+A)+\cos ,(240^{o}-A) $ $ =\cos A+2\cos 240^{o}\cos A $ $ =\cos A{1+2\cos (180^{o}+60^{o})}=\cos A,{ 1+2,( -\frac{1}{2} ) } $ $ =0 $ .
BETA
