b.
\(\Rightarrow\dfrac{x}{2}-60^0=90^0+k180^0\)
\(\Leftrightarrow\dfrac{x}{2}=150^0+k180^0\)
\(\Leftrightarrow x=300^0+k360^0\)
c.
\(\Leftrightarrow cos\left(3x+\dfrac{\pi}{4}\right)=-cos2x\)
\(\Leftrightarrow cos\left(3x+\dfrac{\pi}{4}\right)=cos\left(\pi-2x\right)\)
\(\Leftrightarrow\left[{}\begin{matrix}3x+\dfrac{\pi}{4}=\pi-2x+k2\pi\\3x+\dfrac{\pi}{4}=2x-\pi+k2\pi\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{3\pi}{20}+k2\pi5\\x=-\dfrac{5\pi}{4}+k2\pi\end{matrix}\right.\)
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