[tex]\text{The point corresponding to an angle}\ \alpha\ \text{ on the unit circle}\\\\\text{has the coordinates}\ (\cos\alpha,\ \sin\alpha).\\\\\text{We know}\ \cos(-x)=\cos(x)\ \text{and}\ \sin(-x)=-\sin(x).\\\\\cos\left(-\dfrac{2\pi}{3}\right)=\cos\left(\dfrac{2\pi}{3}\right)=-\cos\left(\dfrac{\pi}{3}\right)=-\dfrac{1}{2}\\\\\sin\left(-\dfrac{2\pi}{3}\right)=-\sin\left(\dfrac{2\pi}{3}\right)=-\sin\left(\dfrac{\pi}{3}\right)=-\dfrac{\sqrt3}{2}\\\\Answer:\ \boxed{C.\ \left(-\dfrac{1}{2};\ -\dfrac{\sqrt3}{2}\right)}[/tex]
