Câu 15: Ý B
\(cos^2x-cosx=0\)\(\Leftrightarrow\left[{}\begin{matrix}cosx=0\\cosx=1\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{2}+k\pi\\x=k2\pi\end{matrix}\right.\)(\(k\in Z\))
Do \(0< x< \pi\Rightarrow\left[{}\begin{matrix}0< \dfrac{\pi}{2}+k\pi< \pi\\0< k2\pi< \pi\end{matrix}\right.\)(\(k\in Z\))\(\Leftrightarrow\left[{}\begin{matrix}-\dfrac{1}{2}< k< \dfrac{1}{2}\\0< k< \dfrac{1}{2}\end{matrix}\right.\)(\(k\in Z\))
\(\Rightarrow\left[{}\begin{matrix}k=0\\k\in\varnothing\end{matrix}\right.\) \(\Rightarrow x=\dfrac{\pi}{2}+0.\pi=\dfrac{\pi}{2}\)
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