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piltalpha-betalt3pi,sinalpha+sinbeta=-21...

`piltalpha-betalt3pi`,`sinalpha+sinbeta=-21/65`,`cosalpha+cosbeta="-27/65`,then `cos((alpha-beta)/2)=` (A) `-6/65` (B) `-3/sqrt(130)` (C) `3/sqrt(130)` (D) `6/65`

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