Posted by **Christine** on Sunday, June 6, 2010 at 9:49pm.

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

cos(165) degrees

- Algebra 2 Honors -
**Reiny**, Sunday, June 6, 2010 at 9:54pm
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

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