If tan y = -4/3 and y is in Q I'VE find cos 2y

Answer 1

well, you know that
siny = 3/5
cos2y = 1 - 2sin^2 y

or, you know that

tan 2y = 2tany/(1-tan^2 y)
and sec^2 2y = 1 + tan^2 2y

good way to check your answer.