13.
\(sinx=\dfrac{1}{2}\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{6}+k2\pi\\x=\dfrac{5\pi}{6}+k2\pi\end{matrix}\right.\)
14.
\(sinx.cosx=\dfrac{1}{2}\Leftrightarrow2sinx.cosx=1\)
\(\Leftrightarrow sin2x=1\Leftrightarrow2x=\dfrac{\pi}{2}+k2\pi\)
\(\Leftrightarrow x=\dfrac{\pi}{4}+k\pi\)
15.
\(sinx=sina\)
\(\Leftrightarrow\left[{}\begin{matrix}x=a+k2\pi\\x=\pi-a+k2\pi\end{matrix}\right.\)
16.
\(2cosx+\sqrt{2}=0\)
\(\Leftrightarrow cosx=-\dfrac{\sqrt{2}}{2}\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{3\pi}{4}+k2\pi\\x=-\dfrac{3\pi}{4}+k2\pi\end{matrix}\right.\)
17.
\(sinx=0\Rightarrow x=k2\pi\) là nghiệm sai (thiếu nghiệm)
18.
\(2sinx+1=0\)
\(\Leftrightarrow sinx=-\dfrac{1}{2}\)
\(\Leftrightarrow\left[{}\begin{matrix}x=-\dfrac{\pi}{6}+k2\pi\\x=\dfrac{7\pi}{6}+k2\pi\end{matrix}\right.\)
