Notice that
1/6 - 1/4 = -1/12
Then
cos(-π/12) = cos(π/6 - π/4)
cos(-π/12) = cos(π/6) cos(π/4) + sin(π/6) sin(π/4)
cos(-π/12) = (√3/2) (1/√2) + (1/2) (1/√2)
cos(-π/12) = (√6 + √2)/4
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