\(sin2x-2sinx=0\)
\(\Leftrightarrow2sinx.cosx-2sinx=0\)
\(\Leftrightarrow sinx\left(cosx-1\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}sinx=0\\cosx=1\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=k\pi\\x=k2\pi\end{matrix}\right.\) \(\Rightarrow x=k\pi\)
\(cosx.cos4x-cosx.cos2x=0\)
\(\Leftrightarrow cosx\left(cos4x-cos2x\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}cosx=0\\cos4x=cos2x\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=\frac{\pi}{2}+k\pi\\4x=2x+k2\pi\\4x=-2x+k2\pi\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=\frac{\pi}{2}+k\pi\\x=k\pi\\x=\frac{k\pi}{3}\end{matrix}\right.\)
\(cos11x.cos3x=cos17x.cos9x\)
\(\Leftrightarrow\frac{1}{2}\left(cos24x+cos8x\right)=\frac{1}{2}\left(cos26x+cos8x\right)\)
\(\Leftrightarrow cos24x=cos26x\)
\(\Rightarrow\left[{}\begin{matrix}26x=24x+k2\pi\\26x=-24x+k2\pi\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=k\pi\\x=\frac{k\pi}{25}\end{matrix}\right.\)
\(sin18x.cos13x=sin9x.cos4x\)
\(\Leftrightarrow\frac{1}{2}\left(sin31x+sin5x\right)=\frac{1}{2}\left(sin13x+sin5x\right)\)
\(\Leftrightarrow sin31x=sin13x\)
\(\Rightarrow\left[{}\begin{matrix}31x=13x+k2\pi\\31x=\pi-13x+k2\pi\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=\frac{k\pi}{9}\\x=\frac{\pi}{44}+\frac{k\pi}{22}\end{matrix}\right.\)
