[tex]\bf f(x)=4cos(2x-\pi )\implies 0=4cos(2x-\pi )\implies 0=cos(2x-\pi )
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cos^{-1}(0)=cos^{-1}[cos(2x-\pi )]\implies cos^{-1}(0)=2x-\pi
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2x-\pi =
\begin{cases}
\frac{\pi }{2}\\
\frac{3\pi }{2}
\end{cases}\\\\
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2x-\pi =\cfrac{\pi }{2}\implies 2x=\cfrac{3\pi }{2}\implies \measuredangle x=\cfrac{3\pi }{4}\\\\
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2x-\pi=\cfrac{3\pi }{2}\implies 2x=\cfrac{5\pi }{2}\implies \measuredangle x=\cfrac{5\pi }{4}[/tex]
