Posted by Christine on .
Find the exact value by using an appropriate sum or difference identity.
cos(165) degrees

Algebra 2 Honors 
Reiny,
For these type of questions, you must split up your given angles into a sum or a difference of angles whose trig ratios you know
e.g. 165 = 120+45
then cos(165)
= cos(120+45)
= cos120cos45  sin120sin45
you should know both sin45 and cos45
for sin120,
sin120 = sin60
cos120 = cos60
take it from here