Đáp án + Giải thích các bước giải:
`7.`
`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)`
`9.`
`tanx-3cotx=2`
ĐKXĐ: `{(cosxne0),(sinxne0):}=>sin2xne0<=>xne(kpi)/2`
`PT<=>tanx-3/tanx=2`
`<=>tan^2x-2tanx-3=0`
`<=>`\(\left[ \begin{array}{l}\tan x=3\\\tan x=-1\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=\arctan3+k\pi\\x=-\dfrac{\pi}{4}+k\pi\end{array} \right.\) `(kinZZ)`
