+ Tính `sin\alpha`
Ta có: `sin^2\alpha+cos^2\alpha=1`
`\to ` `sin^2\alpha=1-cos^2\alpha=1-(-2/5)^2=21/25`
`\to ` `sin\alpha=\sqrt[21]/5`
+ Tính `cot\alpha`
Ta có: Ta có: `cot\alpha=(cos\alpha)/(sin\alpha)=\frac{-2/5}{\sqrt[21]/5}=(-2\sqrt[21])/21`
+ Tính `sin2\alpha`
Ta có: `sin2\alpha=2.sin\alpha.cos\alpha=2.\sqrt[21]/5.(-2/5)=-(4\sqrt[21])/25`
+ Tính `cos2\alpha`
Ta có: `cos2\alpha=cos^2\alpha-sin^2\alpha=(-2/5)^2-(\sqrt[21]/5)^2=-17/25`
+ Tính `tan2\alpha`
`tan\alpha.cot\alpha=1` `\to` `tan\alpha=1/cot\alpha=\sqrt[21]/2`
Ta có: `tan2\alpha=(sin2\alpha)/(cos2\alpha)=\frac{-(4\sqrt[21])/25}{-17/25}=(4\sqrt[21])/17`
