What is the exact value of cosine of open paren 11 pi over 12 close paren ?

Since 11π/12 = π − π/12, cos(11π/12) = −cos(π/12) = −cos(15°). Using the half-angle formula, cos(15°) = √((1 + cos 30°)/2) = √(2 + √3)/2. Therefore the exact value is −√(2 + √3)/2, which is choice A.

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