Given that cos(150°).
[tex]\cos (150^o)=\cos (180^o-30^o)[/tex][tex]\text{Use }\cos (180^o-30^o)=-\cos 30^o[/tex][tex]\cos (150^o)=-\cos (30^o)[/tex][tex]\text{Use }\cos (30^o)=\frac{\sqrt[]{3}}{2}\text{.}[/tex][tex]\cos (150^o)=-\frac{\sqrt[]{3}}{2}[/tex]Hence the required value is
[tex]\cos (150^o)=-\frac{\sqrt[]{3}}{2}[/tex]