Express the Product as a Sum Containing Only Sines or Cosines

Express the product as a sum containing only sines or cosines.
- $\cos ( 5 \theta ) \cos ( 4 \theta )$

A) $\frac { 1 } { 2 } [ \cos ( 9 \theta ) - \cos \theta ]$
B) $\frac { 1 } { 2 } [ \cos \theta + \cos ( 9 \theta ) ]$
C) $\cos ^ { 2 } \left( 20 \theta ^ { 2 } \right)$
D) $\frac { 1 } { 2 } [ \cos ( 9 \theta ) - \sin \theta ]$

Correct Answer:

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