If cos y = -4/5 when 90°≤ y ≤ 180° what is the value of cos 2y?

Answer 1

quadrant 2

cos 2y = 1 - 2 sin^2 y
but sin y = 3/5 (3 4 5 triangle)

cos 2 y = 1 - 2 (3/5)
= 1 - 6/5
= -1/5

Answer 2

last 3 lines should be

cos (2y) = 1 - 2(3/5)^2
= 1 - 18/25 = 7/25 or .28

Answer 3

Whoops, thank you