`~rai~`
$2sin^2x+7sinx-4=0$
$\Leftrightarrow$ \(\left[ \begin{array}{l}sinx=\dfrac{1}{2}(nhận)\\sinx=-4(loại)\end{array} \right.\)
$\Leftrightarrow$\(\left[ \begin{array}{l}x_1=\dfrac{\pi}{6}+k2\pi\\x_2=\dfrac{5\pi}{6}+k2\pi(k\in\mathbb{Z})\end{array} \right.\)
$+)TH1:\dfrac{\pi}{2}<x_1<\dfrac{3\pi}{2}$
$\Leftrightarrow \dfrac{\pi}{2}<\dfrac{\pi}{6}+k2\pi<\dfrac{3\pi}{2}$
$\Leftrightarrow \dfrac{1}{6}<k<\dfrac{2}{3}$
$\text{Do k}\in \mathbb{Z}\Rightarrow k\in\varnothing\Leftrightarrow x\in\varnothing$
$+TH2:\dfrac{\pi}{2}<x_2<\dfrac{3\pi}{2}$
$\Leftrightarrow \dfrac{\pi}{2}<\dfrac{5\pi}{6}+k2\pi<\dfrac{3\pi}{2}$
$\Leftrightarrow -\dfrac{1}{6}<k<\dfrac{1}{3}$
$\text{Do k}\in\mathbb{Z}\Rightarrow k=0$
$\Leftrightarrow x=\dfrac{5\pi}{6}\approx2,62.$
$\text{Vậy x}\approx 2,62.$
