Use a Sum or Difference Identity to Find the Exact $$\cos 9 ^ { \circ } - \cos 33 ^ { \circ }$$

Use a sum or difference identity to find the exact value.
- $$\cos 9 ^ { \circ } - \cos 33 ^ { \circ }$$

A) $- 2 \sin 21 ^ { \circ } \cos \left( - 12 ^ { \circ } \right)$
B) $- 2 \cos 21 ^ { \circ } \sin \left( - 12 ^ { \circ } \right)$
C) $- 2 \cos 21 ^ { \circ } \cos \left( - 12 ^ { \circ } \right)$
D) $- 2 \sin 21 ^ { \circ } \sin \left( - 12 ^ { \circ } \right)$

Correct Answer:

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