`a)` Đúng, vì `tan alpha<0`

`b)` Sai, vì với `alpha in (90^@; 180^@)` thì `cos alpha<0`

`c)` Sai, vì `cot alpha=1/[tan alpha]=1/[[-5]/12]=-12/5`

`d)` Sai

Ta có `-tan alpha=5/12`

`=> tan^2 alpha=25/144`

`=> 1+tan^2 alpha=169/144`

`=> 1/[sin^2 alpha]=169/144`

`=> sin^2 alpha=144/169`

`=> sin alpha=12/13`