## Calculus 10th Edition

$\dfrac{dy}{dx}=-\dfrac{\sqrt{3}}{3}.$
$y=C\cos{x}\rightarrow \dfrac{dy}{dx}=-C\sin{x}.$ When $C=\dfrac{2}{3}$ and $x=\dfrac{\pi}{3}\rightarrow\dfrac{dy}{dx}=-\frac{2}{3}\sin{\frac{\pi}{3}}=-\dfrac{\sqrt{3}}{3}.$