Đáp án+Giải thích các bước giải:
`1+tan^2\alpha=\frac{1}{cos^2\alpha}`
`⇒1+2^2=\frac{1}{cos^2\alpha}`
`⇒\frac{1}{cos^2\alpha}=5`
`⇒cos^2\alpha=\frac{1}{5}`
`⇒cos\alpha=\frac{\sqrt{5}}{5}`
`sin^2alpha+cos^2\alpha=1`
`⇒sin\alpha=\sqrt{1-cos^2\alpha}`
`⇒sin\alpha=\sqrt{1-(\frac{\sqrt{5}}{5})^2}=\frac{2\sqrt{5}}{5}`
`cot\alpha=\frac{1}{tan\alpha}=\frac{1}{2}`
Vậy $\begin{cases}sin\alpha=\dfrac{2\sqrt{5}}{5}\\cos\alpha=\dfrac{\sqrt{5}}{5}\\cot\alpha=\dfrac{1}{2}\end{cases}$
