Use a Double-Angle Formula or Half-Angle Formula to Simplify$\cos ^ { 2 } \frac { \pi } { 8 } - \sin ^ { 2 } \frac { \pi } { 8 }$

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Use a Double-Angle Formula or Half-Angle Formula to Simplify $\cos ^ { 2 } \frac { \pi } { 8 } - \sin ^ { 2 } \frac { \pi } { 8 }$