Plug and chug right into sine's double-angle formula.
Answer
Example 3
Find the exact value of cos (15°).
Hint
You can use the subtraction or half-angle identities on this one.
Answer
Example 4
Find the exact value of sin (112.5°).
Hint
This is a pretty involved problem. 112.5 is half of 225, which equals 180 + 45. Use the addition formula to get cos 225, then plug that into the half-angle formula.
Answer
Example 5
Find the exact value of tan (-315°).
Hint
Tangent is an odd function, so the result tan (-315°) will be -tan (315°). Careful, though, because 315° is in Quadrant IV.
and 
, find the exact value of sin (2α).
