Ta có : `1+tan^2alpha=1/(cos^2alpha)`
`<=> 1+2^2=1/(cos^2alpha)`
`<=> 5=1/(cos^2alpha)`
`<=> 5 cos^2alpha=1`
`<=> cos^2alpha=1/5`
`<=> cos alpha=+-1/(sqrt5)`
+) Với `cos alpha=1/(sqrt5)`
Ta có : `tan alpha=(sin alpha)/(cos alpha)`
`to sin alpha=tan alpha.cos alpha`
`to sin alpha=2 . 1/(sqrt5)=(2sqrt5)/5`
+) Với `cos alpha=-1/(sqrt5)`
Ta có : `tan alpha=(sin alpha)/(cos alpha)`
`to sin alpha=tan alpha.cos alpha`
`to sin alpha=2 . (-1/(sqrt5))=-(2sqrt5)/5`
