`ĐK: D = RR \\ {π/2 + kπ | k ∈ ZZ}`
`2tan^2 x + 3/(cos x) = -3`
`<=> (2sin^2 x)/(cos^2 x) + 3/(cos x) = -3`
`=> 2sin^2 x + 3cos x = -3cos x`
`<=> 2 - 2cos^2 x + 6cos x = 0`
`<=>` \(\left[ \begin{array}{l}cos x = \dfrac{3 + \sqrt{13}}{2} (l)\\cos x = \dfrac{3 - \sqrt{13}}{2}\end{array} \right.\)
`<=> x = +-arccos ((3 - \sqrt{13})/2) + k2π` `(k ∈ ZZ)`