Đáp án+ Giải thích các bước giải:
`i.`
`2cos2x-8cosx+5=0`
`<=>2(2cos^2x-1)-8cosx+5=0`
`<=>4cos^2x-8cosx+3=0`
`<=>`\(\left[ \begin{array}{l}\cos x=\dfrac{3}{2}(VN)\\\cos x=\dfrac{1}{2}\end{array} \right.\)
`<=>x=+-pi/3+k2pi(kinZZ)`
`j.`
`2(sin^4x+cos^4x)=2sin2x-1`
`<=>2[(sin^2x+cos^2x)^2-2sin^2xcos^2x]=2sin2x-1`
`<=>2(1-2sin^2xcos^2x)=2sin2x-1`
`<=>2-sin^2 2x=2sin2x-1`
`<=>sin^2 2x+2sin2x-3=0`
`<=>`\(\left[ \begin{array}{l}\sin 2x=1\\\sin 2x=-3(VN)\end{array} \right.\)
`<=>2x=pi/2+k2pi`
`<=>x=pi/4+kpi(kinZZ)`
