Tìm x biết 4x^2 - 49 = 0

$4x^{2} - 49 = 0$
⇔  $(2x)^{2} - $ $7^{2} = 0 $ 
⇔ $ (2x - 7)(2x+7) = 0^{}$

⇔ $\left[\begin{matrix} 2x + 7 = 0 \\ 2x - 7 = 0\end{matrix}\right.$

⇔ $\left[\begin{matrix} 2x = 0 - 7\\ 2x = 0+7\end{matrix}\right.$

⇔ $\left[\begin{matrix} 2x = -7\\ 2x = 7\end{matrix}\right.$

⇔ $\left[\begin{matrix} x = -7 : 2\\ x = 7 : 2\end{matrix}\right.$

⇔ $\left[\begin{matrix} x = -3,5\\ x = 3,5\end{matrix}\right.$

$\text{Vậy x }$ $\in$ { $\text{-3,5 ; 3,5}$}