Question #872b5

1 Answer
Oct 1, 2017

If cosa=-1/3, a must be between 90 and 180 degrees. In other words, a in 2nd quadrant in unit circle. In this quadrant sina>0. So, if cosa=-1/3, sina=(2sqrt(2))/3

After using sin2a=2sina*cosa identity, I found sin2a=-(4sqrt(2))/9

Explanation:

1) I found sina after being given cosu=-1/3 and 90 < a < 180 conditions.

2) I used sin2a=2sina*cosa identity.