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If cos(x-y) , cos x and cos (x+y) are in H.P. , then cos x. sec `(y/2)` is equal to :

A

1

B

2

C

`sqrt2`

D

none of these

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The correct Answer is:
C
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OBJECTIVE RD SHARMA ENGLISH-TRIGONOMETRIC RATIOS AND IDENTITIES-Exercise
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  9. about to only mathematics

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