Use a Double-Angle Formula to Find the Exact Value of Cos2u

Multiple Choice

Use a double-angle formula to find the exact value of cos2u when​ $$\sin u = \frac { 7 } { 25 } \text {, where } \frac { \pi } { 2 } < u < \pi$$

A) $\cos 2 u = \frac { 527 } { 625 }$
B) ​ $\cos 2 u = - \frac { 1152 } { 625 }$
C) ​ $\cos 2 u = \frac { 336 } { 625 }$
D) ​ $\cos 2 u = \frac { 168 } { 625 }$
E) ​ $\cos 2 u = - \frac { 478 } { 625 }$