Use the Sum-To-Product Formulas to Select the Sum or Difference $$\cos ( 5 \phi + 2 \pi ) + \cos 5 \phi$$

Use the sum-to-product formulas to select the sum or difference as a product.​ $$\cos ( 5 \phi + 2 \pi ) + \cos 5 \phi$$ ​

A) ​ $- 2 \cos 5 \phi \cos \pi$
B) ​ $- 2 \cos 5 \pi$
C) $2 \cos ( 5 \phi + \pi ) \cos \pi$
D) ​ $2 \cos ( \phi + \pi ) \cos \pi$
E) ​ $2 \cos ( 5 \phi + \pi )$

Correct Answer:

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