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If cos alpha + cos beta =cos ""(3pi)/(7)...

If `cos alpha + cos beta =cos ""(3pi)/(7)and sin alpha + sin beta = sin ""(3pi)/(7),` then the value of `cos ^(2)""(alpha-beta)/(2) ` is-

A

`1/4`

B

`1/2`

C

`(1)/(sqrt2)`

D

`1/8`

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The correct Answer is:
A
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