Use identities to find values of the sine and cosine functions of the function for the angle measure.
\( 2 x \) , given \( \tan x = - 4 \) and \( \cos x > 0 \)
\( \cos 2 x = \frac { - 15} { 17} \)
(Simplify your answer, including any radicalsf Use integers or fractions for any numbers in the expression.)
\( \sin 2 x = \square \)
(Simplify your answer, including any radicals. Use integers or fractions for any numbers in the expression.)