Biết cos alpha = -2/3 và π < alpha < 3π /2 . Tính sin alpha , tan alpha , cot alpha .

Vì `pi<alpha<(3pi)/2`

`→sinalpha<0`

`tanalpha>0`

`cotalpha>0`

`sin^2alpha+cos^2alpha=1`

`→sin^2alpha=1-(-2/3)^2`

`→sin^2alpha=5/9`

`→sinalpha=-sqrt(5)/3`

`tanalpha=sinalpha/cosalpha=sqrt(5)/2`

`cotalpha=1/tanalpha=(2sqrt(5))/5`