Evaluate giving exact values and identities. Calculator based solutions are not acceptable. Thanks for any help
cos (-5pi/12)

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  1. That is -75 degrees. Use the formula for cos (A + B), where A = -45 degrees and B = -30 degrees.

    cos (-5pi/12) = cos (5pi/12)
    = cos45deg*cos30deg - sin45deg*sin30deg
    = (sqrt2)/2*(sqrt3)/2 - sqrt2/4
    = [(sqrt6)- (sqrt2)]/4
    = sqrt2*(sqrt3 -1)/4

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