Use a Double-Angle Formula to Find the Exact Value Of \cos

Use a double-angle formula to find the exact value of $\cos 2 u$ when $\sin u = \frac { 5 } { 13 }$ , where $\frac { \pi } { 2 } < u < \pi$ .

A) $\cos 2 u = - \frac { 94 } { 169 }$
B) $\quad \cos 2 u = \frac { 60 } { 169 }$
C) $\quad \cos 2 u = \frac { 120 } { 169 }$
D) $\cos 2 u = \frac { 119 } { 169 }$
E) $\cos 2 u = - \frac { 288 } { 169 }$

Correct Answer:

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