Đáp án:
`18A`
`22B`
Giải thích các bước giải:
`tan\ \alpha=1/2`
`tan\ \alpha . cot\ \alpha=1`
`⇒ cot\ \alpha = \frac{1}{tan\ \alpha}=2`
`1+cot^2 \alpha=\frac{1}{sin^2 \alpha}`
`⇔ 1+4=\frac{1}{sin^2 \alpha}`
`⇔ 5=\frac{1}{sin^2 \alpha}`
`⇔ sin^2 \alpha=1/5`
`⇒ sin\ \alpha=\frac{1}{\sqrt{5}}`
Vậy `sin\ \alpha=\frac{1}{\sqrt{5}}`
22/
`cos\ \alpha=\frac{\sqrt{2}}{2}`
`cos^2 \alpha+sin^2 \alpha=1`
`⇔ (\frac{\sqrt{2}}{2})^2+sin^2 \alpha=1`
`⇒ sin\ \alpha=\sqrt{1-(\frac{\sqrt{2}}{2})^2}=\frac{\sqrt{2}}{2}`
Vậy `sin\ \alpha=\frac{\sqrt{2}}{2}`
