`2tanx ² +3tanx +1=0` `(1)`
`ĐK: cosx \ne0`
`⇔x\ne π/2 + kπ`
`(1)⇔2tanx²+2tanx+tanx+1=0`
`⇔2tanx(tanx+1) + (tanx +1)=0`
`⇔(tanx+1)(2tanx+1)=0`
`⇔` $\left[\begin{matrix} tanx=-1\\ tanx=\dfrac{-1}{2}\end{matrix}\right.$
`⇔` $\left[\begin{matrix} x=\dfrac{-π}{4} +kπ\\ x=arctan\dfrac{-1}{2}+kπ\end{matrix}\right.$
`Vậy ` $S={\dfrac{-π}{4} +kπ; arctan\dfrac{-1}{2}+kπ}$
