Asked by Christine

Find the exact value by using an appropriate sum or difference identity.

cos(165) degrees
Answered by 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