ANSWER
[tex]\begin{equation*} -\frac{\sqrt{3}}{2} \end{equation*}[/tex]EXPLANATION
We want to find the value of:
[tex]\cos210[/tex]We can rewrite it as:
[tex]\cos(180+30)[/tex]Because 210 degrees is in the third quadrant, the value of its cosine is negative and it is the same magnitude as the cosine of 30 degrees.
In other words, the value of cos210 is:
[tex]\cos(210)=-\cos(30)=-\frac{\sqrt{3}}{2}[/tex]That is the answer.
