SOLUTION
We are told to find
[tex]\cos 210^o[/tex]
210 degrees is in the third quadrant. In the third quadrant only
[tex]\begin{gathered} \tan \theta\text{ is positive} \\ \cos \theta\text{ and sin }\theta\text{ is negative} \end{gathered}[/tex]
Since
[tex]\begin{gathered} In\text{ the third quadrant, the angle is }\theta-180^0 \\ \\ \cos 210^o=-\cos (210-180) \\ \\ =-\cos 30^o \\ \\ =-\frac{\sqrt[]{3}}{2} \end{gathered}[/tex]
