C-08-Q188mediumsingle_mcqIf cosα=−12\cos\alpha = -\dfrac{1}{2}cosα=−21 and π2<α<π\dfrac{\pi}{2} < \alpha < \pi2π<α<π, then α\alphaα equals —aπ3\dfrac{\pi}{3}3πb2π3\dfrac{2\pi}{3}3