Use a Product-To-Sum Formula to Convert the Expression to a Sum

question 17

Multiple Choice

Use a product-to-sum formula to convert the expression to a sum or difference. Simplify where possible. $\cos \frac { \pi } { 5 } \cos \frac { 4 \pi } { 5 }$

• A) \(\frac { 1 } { 2 } \left( \cos \frac { 3 \pi } { 5 } + 1 \right)\)
• B) \(\frac { 1 } { 2 } \left( \cos \frac { 3 \pi } { 5 } - 1 \right)\)
• C) \(1 + \sin ^ { 2 } \frac { \pi } { 5 }\)
• D) \(- \frac { 1 } { 2 } \sin \frac { 3 \pi } { 5 }\)
• E) \(1 - \sin ^ { 2 } \frac { \pi } { 5 }\)

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