cứu kkkkkk

$\displaystyle a,\ y=\frac{1}{1+cosx}$
Hàm số xác định khi và chỉ khi 
$\displaystyle  \begin{array}{{>{\displaystyle}l}}
1+cosx\neq 0\\
\Leftrightarrow cosx\neq -1\\
\Leftrightarrow x\neq \pi +k2\pi 
\end{array}$
$\displaystyle  \begin{array}{{>{\displaystyle}l}}
b,\ sin\left( 3x+\frac{\pi }{6}\right) =\frac{\sqrt{3}}{2}\\
\Leftrightarrow sin\left( 3x+\frac{\pi }{6}\right) =sin\left(\frac{\pi }{3}\right)\\
\Leftrightarrow \left[ \begin{array}{l l}
3x+\frac{\pi }{6} =\frac{\pi }{3} +k2\pi  & \\
3x+\frac{\pi }{6} =\pi -\frac{\pi }{3} +k2\pi  & 
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l l}
3x=\frac{\pi }{6} +k2\pi  & \\
3x=\frac{\pi }{2} +k2\pi  & 
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l l}
x=\frac{\pi }{18} +\frac{2k}{3} \pi  & \\
x=\frac{\pi }{6} +\frac{2k}{3} \pi  & 
\end{array} \right.
\end{array}$