Let alpha and beta be such that pi<alpha...

Let `alpha and 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) `-3/(sqrt(130))` (b) `3/(sqrt(130))` (c) `6/(25)` (d) `6/(65)`

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