Cos(pi/12) solve using half angle formula

Answer 1

cos 2A = 2cos^2 A - 1
cos (π/6) = 2cos^2 (π/12) - 1
√3/2 + 1 = 2cos^2 (π/12)
cos π/12 = ±√(√3/4 + 1/2)

but /12 is in quadrant I
so cos (π/12) = √(√3/4 + 1/2)
or (1/2)√(√3 + 2)