a: \(cos3x=cos\left(\dfrac{4\Omega}{7}\right)\)
=>\(\left[{}\begin{matrix}3x=\dfrac{4}{7}\Omega+k2\Omega\\3x=-\dfrac{4}{7}\Omega+k2\Omega\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{4}{21}\Omega+\dfrac{k2\Omega}{3}\\x=-\dfrac{4}{21}\Omega+\dfrac{k2\Omega}{3}\end{matrix}\right.\)
d: \(cosx+cos3x=0\)
=>\(2\cdot cos\left(\dfrac{3x-x}{2}\right)\cdot cos\left(\dfrac{3x+x}{2}\right)=0\)
=>\(cosx\cdot cos2x=0\)
=>\(\left[{}\begin{matrix}cosx=0\\cos2x=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\Omega}{2}+k2\Omega\\2x=\dfrac{\Omega}{2}+k2\Omega\end{matrix}\right.\Leftrightarrow x=\dfrac{\Omega}{4}+k\Omega\)
