cos x = - $$\frac{1}{2}$$ $$\\$$ <=> cos x = cos $$\frac{2\pi}{3}$$ $$\\$$ <=> x = $$\pm$$ $$\frac{2\pi}{3}$$ + k.2$$\pi$$ (K $$\in$$ $$\mathbb{Z}$$)

Cos x = -1/2
⇔ Cos x = Cos ( (2pi)/3)
⇔ [(x = (2pi)/3 + k2pi ),( x = -(2pi)/3 + k2pi ):} ( k ∈ Z ) 
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