Write the Expression as the Sine, Cosine, or Tangent of a Double

Multiple Choice

Write the expression as the sine, cosine, or tangent of a double angle. Then find the exact value of the expression.
- $$\cos ^ { 2 } 60 ^ { \circ } - \sin ^ { 2 } 60 ^ { \circ }$$

A) $- \frac { 1 } { 2 }$
B) $\frac { \sqrt { 3 } } { 2 }$
C) $- \frac { \sqrt { 3 } } { 2 }$
D) $\frac { 1 } { 2 }$