1/(n(n+1)) = 1/n - 1/(n+1) hence
cos(π/12) = cos(π/(3*4)) = cos(π/3 - π/4)
Then use the difference formula. cos(a-b) = cos(a)*cos(b) + sin(a)*sin(b)
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1/(n(n+1)) = 1/n - 1/(n+1) hence
cos(π/12) = cos(π/(3*4)) = cos(π/3 - π/4)
Then use the difference formula. cos(a-b) = cos(a)*cos(b) + sin(a)*sin(b)