Question

Question #872b5

Answer 1

If `cosa=-1/3`, `a` must be between `90` and `180` degrees. In other words, a in `2`nd 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.