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Let alpha,beta be such that pi<(alpha-be...

Let `alpha,beta` be such that `pi<(alpha-beta)<3pi`. If `sinalpha+sinbeta=-21/65` and `cosalpha+cosbeta=-27/65`, then the value of `cos((alpha-beta)/2)` is

A

`-6/65`

B

`3/sqrt130`

C

`6/65`

D

`-3/sqrt130`

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