B
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giải pt:
\[4{\cos ^4}x - c{\rm{os}}2x - \frac{1}{2}c{\rm{os}}4x + c{\rm{os}}\frac{{3x}}{4} = \frac{7}{2}\]
\[4{\cos ^4}x - c{\rm{os}}2x - \frac{1}{2}c{\rm{os}}4x + c{\rm{os}}\frac{{3x}}{4} = \frac{7}{2}\]