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If `alpha, beta` be such that `pi < alpha- beta < 3 pi`. If `sin alpha +sin beta= -21/65` and `cos alpha + cos beta = -27/65` , then the value of `cos frac (alpha-beta)(2)` is :

A

`-3/ sqrt130`

B

`3/sqrt130`

C

`6/65`

D

`-6/65`.

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