17.
\(cosx=-\dfrac{\sqrt{2}}{2}\Rightarrow\left[{}\begin{matrix}x=\dfrac{3\pi}{4}+k2\pi\\x=\dfrac{-3\pi}{4}+k2\pi\end{matrix}\right.\)
18.
\(cosx=-\dfrac{\sqrt{3}}{2}\Rightarrow x=\pm\dfrac{5\pi}{6}+k2\pi\)
19.
\(cos\left(x+\dfrac{\pi}{3}\right)=\dfrac{1}{\sqrt{2}}\Rightarrow\left[{}\begin{matrix}x+\dfrac{\pi}{3}=\dfrac{\pi}{4}+k2\pi\\x+\dfrac{\pi}{3}=-\dfrac{\pi}{4}+k2\pi\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=-\dfrac{\pi}{12}+k2\pi\\x=-\dfrac{7\pi}{12}+k2\pi\end{matrix}\right.\) \(\Rightarrow\left[{}\begin{matrix}x=\dfrac{23\pi}{12}\\x=\dfrac{17\pi}{12}\end{matrix}\right.\) có 2 nghiệm
20.
\(cosx=\dfrac{\sqrt{3}}{2}\Rightarrow x=\pm\dfrac{\pi}{6}+k2\pi\)
