cos2C+(1/2)cos2C-(√3/2)sin2C=-3/2 ==> (3/2)cos2C-(√3/2)sin2C=-3/2 ==> √3×[(√3/2)cos2C-(1/2)sin2C]=-3/2 ==> (√3/2)cos2C-(1/2)sin2C=-√3/2 ==> cos(π/6)cos2C-sin(π/6)sin2C=-√3/2 ==> cos[(π/6)+2C]=-√3/2
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