Edit: Added some tex tags
This is a question on a review I am doing, my answer doesn't match up with the answer key and I am looking for some input on what I am doing wrong.
If with in quadrant IV find:
I solved for and
Using negative angle identities and the double angle identity I end up with:
Plugging in my values gives me:
The answer key gives:
Not sure where I am messing up =\ any help is much appreciated.