Question

cos u= -(2/3), pi/2 12 years ago

Answer 1

first determine cos2u
using \[\cos2u = 2\cos^{2}u - 1\]

Answer 2

then sin2u
using \[\sin^{2}2u + \cos^{2}2u = 1\]
and last but not least
\[\tan2u= \frac{ \sin2u }{ \cos2u }\]