Find the value of cos ${210}^{\circ }$

Finding the value of$\cos {210}^{\circ }$

$\cos {210}^{\circ }$can be expressed as

$\cos {210}^{\circ }=\cos ({180}^{\circ }+{30}^{\circ })$ $\left[\because cos(180+x)=-cosx\right]$

$=-\cos {30}^{\circ }$

$=-\frac{\sqrt{3}}{2}$

Hence the value of $\cos {210}^{\circ }$ is $-\frac{\sqrt{3}}{2}$