a/ \(\frac{\pi}{2}< a< \pi\Rightarrow cosa< 0\)
\(\Rightarrow cosa=-\sqrt{1-sin^2a}=-\frac{2\sqrt{6}}{5}\)
\(tanx=\frac{sinx}{cosx}=-\frac{\sqrt{6}}{12}\) ; \(cotx=\frac{1}{tanx}=-2\sqrt{6}\)
b/ \(\frac{3\pi}{2}< a< 2\pi\Rightarrow cosa>0\)
\(\Rightarrow cosa=\frac{1}{\sqrt{1+tan^2a}}=\frac{5\sqrt{26}}{26}\)
\(sina=tana.cosa=-\frac{\sqrt{26}}{26}\)
c/ \(0< a< \frac{\pi}{2}\Rightarrow sina;cosa>0\)
\(\left\{{}\begin{matrix}cos^2a+sin^2a=1\\2sina.cosa=\frac{2}{3}\end{matrix}\right.\)
\(\Rightarrow sina+cosa=\frac{\sqrt{15}}{3}\Rightarrow cosa=\frac{\sqrt{15}}{3}-sina\)
\(\Rightarrow sina\left(\frac{\sqrt{15}}{3}-sina\right)=\frac{1}{3}\Rightarrow sin^2a-\frac{\sqrt{15}}{3}sina+\frac{1}{3}=0\)
\(\Rightarrow\left[{}\begin{matrix}sina=\frac{\sqrt{15}+\sqrt{3}}{6}\Rightarrow cosa=\frac{\sqrt{15}-\sqrt{3}}{6}\\sina=\frac{\sqrt{15}-\sqrt{3}}{6}\Rightarrow cosa=\frac{\sqrt{15}+\sqrt{3}}{6}\end{matrix}\right.\) \(\Rightarrow tana=\frac{sina}{cosa}=...\)
d/ \(\frac{\pi}{2}< a< \pi\Rightarrow\left\{{}\begin{matrix}sina>0\\cosa< 0\end{matrix}\right.\)
\(cosa=\sqrt{2}-sina\) \(\Rightarrow sin^2a+\left(\sqrt{2}-sina\right)^2=1\)
\(\Leftrightarrow2sin^2a-2\sqrt{2}sina+1=0\Rightarrow sina=\frac{\sqrt{2}}{2}\)
\(\Rightarrow cosa=-\sqrt{1-sin^2a}=-\frac{\sqrt{2}}{2}\)
\(tana=\frac{sina}{cosa}=-1\)
a, \(sin\alpha=\frac{1}{5},\frac{\pi}{2}< \alpha< \pi\)
+) \(sin^2\alpha+cos^2\alpha=1\)
\(\Leftrightarrow\left(\frac{1}{5}\right)^2+cos^2\alpha=1\Leftrightarrow cos^2\alpha=\frac{24}{25}\Leftrightarrow cos\alpha=\pm\frac{2\sqrt{6}}{5}\)
mà \(\frac{\pi}{2}< \alpha< \pi\Rightarrow cos\alpha=-\frac{2\sqrt{6}}{5}\)
+) \(tan\alpha=\frac{sin\alpha}{cos\alpha}=\frac{\frac{1}{5}}{-\frac{2\sqrt{6}}{5}}=-\frac{\sqrt{6}}{12}\)
+) \(cot\alpha=\frac{cos\alpha}{sin\alpha}=\frac{-\frac{2\sqrt{6}}{5}}{\frac{1}{5}}=-2\sqrt{6}\)
