a. vì $\pi$ /2<$\alpha$ <$\pi$
⇒ cos $\alpha$ <0
Ta có sin²$\alpha$ +cos²$\alpha$ =1
⇒ cos²$\alpha$ =$\frac{16}{25}$ ⇒cos $\alpha$ =±4/5
Mà cos$\alpha$ <0 ⇒ cos$\alpha$ =-4/5
Có sin2$\alpha$ =2sin$\alpha$cos$\alpha$=2*$\frac{3}{5}$* $\frac{-4}{5}$ =$\frac{-24}{25}$
cos2$\alpha$ =cos²$\alpha$-sin²$\alpha$ =$\frac{16}{25}$- $\frac{9}{25}$= $\frac{7}{25}$
tan2$\alpha$ =sin2$\alpha$ /cos2$\alpha$ =$\frac{-24}{25}$ :$\frac{7}{25}$=$\frac{-24}{7}$
