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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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