If cos(theta-alpha),costheta and cos(th...

If `cos(theta-alpha),costheta and cos(theta+alpha)` are in H.P ., then `costheta"sec"(alpha)/(2)` is equal to

A

`+-sqrt(2)`

B

`+-sqrt(3)`

C

`+-1//sqrt(2)`

D

None of these

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The correct Answer is:

A

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