Use a double-angle formula to find the exact value of $\cos 2 u$ when $\sin u = \frac { 8 } { 17 }$, where $\frac { \pi } { 2 }

Question

Use a double-angle formula to find the exact value of $\cos 2 u$ when $\sin u = \frac { 8 } { 17 }$, where $\frac { \pi } { 2 } < u < \pi$.
A) $\cos 2 u = - \frac { 97 } { 289 }$
B) $\cos 2 u = \frac { 120 } { 289 }$
C) $\cos 2 u = \frac { 240 } { 289 }$
D) $\cos 2 u = \frac { 161 } { 289 }$
E) $\cos 2 u = - \frac { 450 } { 289 }$

Quizplus · Accepted Answer

The Answer of Use a double-angle formula to find the exact value of $\cos 2 u$ when $\sin u = \frac { 8 } { 17 }$, where $\frac { \pi } { 2 } < u < \pi$.
A) $\cos 2 u = - \frac { 97 } { 289 }$
B) $\cos 2 u = \frac { 120 } { 289 }$
C) $\cos 2 u = \frac { 240 } { 289 }$
D) $\cos 2 u = \frac { 161 } { 289 }$
E) $\cos 2 u = - \frac { 450 } { 289 }$

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