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1, sin^4x + cos^4x=cos4x \Leftrightarrow 1-\frac{1}{2}sin^{2}2x=1-2sin^{2}2x \Leftrightarrow sin^{2}2x=0 2, 2 cos^2x+5sinx-4=0 \Leftrightarrow 1+cos2x+5sinx-4=0 \Leftrightarrow 1-2sin^{2}x+5sinx-3=0 \Leftrightarrow 2sin^{2}x -5sinx-2=0 \Leftrightarrow sinx=\frac{5+\sqrt{41}}{4} hoặc sinx=\frac{5-\sqrt{41}}{4} 3,2cos2x-8cosx+5=0 \Leftrightarrow 2(2cos^{2}x-1)-8cosx+5=0 \Leftrightarrow 4cosx^{2}x-8cosx+3=0 4, 5 tanx-2cotx - 3 = 0 \Leftrightarrow 5tan^{2}x-3tanx-2=0
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