Câu 18 neeee

a,
$\displaystyle  \begin{array}{{>{\displaystyle}l}}
sinx=\frac{1}{4}\\
\Leftrightarrow \left[ \begin{array}{l l}
x=arcsin\left(\frac{1}{4}\right) +k2\pi  & \\
x=\pi -arcsin\left(\frac{1}{4}\right) +k2\pi  & 
\end{array} \right.( k\in \mathbb{Z})
\end{array}$
b,
$\displaystyle  \begin{array}{{>{\displaystyle}l}}
cos^{2} x=\frac{2}{5}\\
\Leftrightarrow \left[ \begin{array}{l l}
cosx=\frac{\sqrt{10}}{5} & \\
cosx=\frac{-\sqrt{10}}{5} & 
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l l}
\left[ \begin{array}{l l}
x=arccos\left(\frac{\sqrt{10}}{5}\right) +k2\pi  & \\
x=-arccos\left(\frac{\sqrt{10}}{5}\right) +k2\pi  & 
\end{array} \right. & \\
\left[ \begin{array}{l l}
x=arccos\left(\frac{-\sqrt{10}}{5}\right) +k2\pi  & \\
x=-arccos\left(\frac{-\sqrt{10}}{5}\right) +k2\pi  & 
\end{array} \right. & 
\end{array} \right.( k\in \mathbb{Z})
\end{array}$
c,
$\displaystyle  \begin{array}{{>{\displaystyle}l}}
3tanx+1=0\\
\Leftrightarrow tanx=\frac{-1}{3}\\
\Leftrightarrow x=arctan\left(\frac{-1}{3}\right) +k\pi \ ( k\in \mathbb{Z}) \ 
\end{array}$