which cofunction identities would u use
cos (x+pi/2)=-sinx or sin (x+pi/2)=cosx? how did u determine which one to use?
$\displaystyle \sin (\frac{\pi}{6}) = \frac{1}{2}$
$\displaystyle \sin (\theta) = \cos (\frac{\pi}{2} - \theta)$
Hence, $\displaystyle \cos (\frac{\pi}{3}) = \frac{1}{2}$
Also, $\displaystyle \cos (\pi - \theta) = -\cos (\theta)$
Therefore, $\displaystyle \cos(\frac{2\pi}{3}) = \cos (\pi - \frac{\pi}{3}) = -\cos (\frac{\pi}{3}) = -\frac{1}{2}$