Identify angle with cosine: Identify the angle whose cosine is −21. Using trigonometric identities or a unit circle, we know cos(120°)=−21 and cos(240°)=−21.
Convert angles to radians: Convert the angles to radians because most mathematical problems require answers in radians.120°=120×(π/180)=2π/3 radians,240°=240×(π/180)=4π/3 radians.
Consider periodic nature: Consider the periodic nature of the cosine function. Cosine has a period of 2π, so the general solutions are: x=32π+2πk and x=34π+2πk, where k is any integer.
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