Question

find the exact value by using half angle identities.

cos165°

Answer 1

http://answers.yahoo.com/question/index?qid=20080111200806AAqyq89

Answer 2

1 - cos(330°)
------------------- = sin² 165°
......2

1 - √(3)/2
------------- = sin² 165°
......2

.0670 = sin² 165°
.2588 = sin 165° <== ANSWER
Remember that sine values in the second quadrant are positive.

1 + cos(330°)
------------------- = cos² 165°
......2

1 + √(3)/2
-------------- = cos² 165°
......2

.9330 = cos² 165°
-.9659 = cos 165° <== ANSWER

Answer 3

that fine?

Answer 4

from this, you can say
\[\cos(165) = -\cos(15) = -\cos(\frac{30}{2})\]

Answer 5

Thanks to both! :D