sin a+cos a= 2/3. tính sin 2a

`sin \alpha +cos \alpha =2/3`

`=>(sin \alpha +cos \alpha)^2 =(2/3)^2`

`<=>sin^2 \alpha +cos^2 \alpha +2cos \alpha sin \alpha =4/9`

`<=>1+sin 2\alpha =4/9`

`<=>sin 2\alpha =-5/9`

Vậy `sin 2\alpha =-5/9`