Simplify the Expression Algebraically $$\cos ( 7 x + 4 y ) \cos ( 7 x - 4 y )$$

Simplify the expression algebraically.​ $$\cos ( 7 x + 4 y ) \cos ( 7 x - 4 y )$$ ​

A) $\cos ^ { 2 } 7 x - \sin ^ { 2 } y$
B) $\cos ^ { 2 } 7 x + \sin ^ { 2 } 4 y$
C) $\cos ^ { 2 } 7 x - \sin ^ { 2 } 4 y$
D) $\cos ^ { 2 } x - \sin ^ { 2 } 4 y$
E) $\cos ^ { 2 } x - \sin ^ { 2 } y$

Correct Answer:

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