Find the exact value of the expression.
-$\cos 840 ^ { \circ }$
A) $- \frac { \sqrt { 3 } } { 2 }$
B) $\frac { \sqrt { 2 } } { 2 }$
C) $- \sqrt { 3 }$
D) $- \frac { 1 } { 2 }$

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The Answer of Find the exact value of the expression.
-$\cos 840 ^ { \circ }$
A) $- \frac { \sqrt { 3 } } { 2 }$
B) $\frac { \sqrt { 2 } } { 2 }$
C) $- \sqrt { 3 }$
D) $- \frac { 1 } { 2 }$

Find the Exact Value of the Expression cos⁡840∘\cos 840 ^ { \circ }cos840∘

Find the Exact Value of the Expression $\cos 840 ^ { \circ }$