Giải:4sin³xcos3x + 4cos³xsin3x + 3√3.cos4x = 3
↔(3sinx-sin3x) cos3x + (cos3x+3cosx)sin3x + 3√3.cos4x = 3
↔3sinxcos3x+3cosx sin3x+3√3.cos4x = 3
↔(sinxcos3x+cosx sin3x)+√3.cos4x = 1
↔sin4x+√3.cos4x = 1
↔1/2sin4x+√3/2.cos4x = 1/2
Đặt cosα=√3/2 ; sinα=1/2 →α= π/6
↔sin π/6.sin4x+cos π/6.cos4x=1/2
↔cos(4x- π/6)=cos π/3
↔x= π/8+kπ/2 Hoặc x=- π/24+ kπ/2
trigonometric,math-online
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