Toán Lớp 11: giải phương trình cos (x + pi/6)=-1/2

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1. $$\begin{gathered} \cos \left(x+\frac{\mathrm{\pi }}{6}\right)=–\frac{1}{2} \\ <=>\cos \left(x+\frac{\mathrm{\pi }}{6}\right)=\cos \frac{2\mathrm{\pi }}{3} \\ <=>x+\frac{\mathrm{\pi }}{6}=\frac{2\mathrm{\pi }}{3}+k2\mathrm{\pi } \mathrm{hoặc} \mathrm{x}+\frac{\mathrm{\pi }}{6}=–\frac{2\mathrm{\pi }}{3}+\mathrm{k}2\mathrm{\pi }, \mathrm{k}\in \mathrm{Z} \\ <=>\mathrm{x}=\frac{\mathrm{\pi }}{2}+\mathrm{k}2\mathrm{\pi } \mathrm{hoặc} \mathrm{x}=–\frac{5\mathrm{\pi }}{6}+\mathrm{k}2\mathrm{\pi }, \mathrm{k}\in \mathrm{Z} \\ \mathrm{Vậy} \mathrm{S}=\left\{\frac{\mathrm{\pi }}{2}+\mathrm{k}2\mathrm{\pi }; –\frac{5\mathrm{\pi }}{6}+\mathrm{k}2\mathrm{\pi }\right\}, \mathrm{k}\in \mathrm{Z} \end{gathered}$$

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