Question

If sin A= radical five over three, then cos 2A equals

Answer 1

Using the trig identity: cos(2A) = cos²A − sin²A = 1 − 2 sin²A , we can plug in what we already know:
1 - 2 ((sqrt 5)/3)^2 = 1- 2(5/9) = 1- 10/9 = -1/9