`a,`
`25x^{2} - 9 = 0`
$\Leftrightarrow$ `5x^{2} - 3^{2} = 0`
$\Leftrightarrow$ `(5x - 3)(5x + 3) = 0`
$\Leftrightarrow$ $\left[\begin{matrix} 5x - 3 = 0\\ 5x + 3 = 0\end{matrix}\right.$
$\Leftrightarrow$ $\left[\begin{matrix} 5x = 0 + 3\\ 5x = 0 - 3\end{matrix}\right.$
$\Leftrightarrow$ $\left[\begin{matrix} 5x = 3\\ 5x = -3\end{matrix}\right.$
$\Leftrightarrow$ $\left[\begin{matrix} x = 3 : 5\\ x = (-3) : 5\end{matrix}\right.$
$\Leftrightarrow$ $\left[\begin{matrix} x = 0,6\\ x = -0,6\end{matrix}\right.$
$\text{Vậy}$ `x` $\in$ `{0,6 ; -0,6}`
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`b,`
` (x-3)^{2} - 4 = 0`
$\Leftrightarrow$ ` (x-3-2)(x-3+2) = 0`
$\Leftrightarrow$ ` (x-5)(x-1) = 0`
$\Leftrightarrow$ $\left[\begin{matrix} x - 5 = 0\\ x - 1 = 0\end{matrix}\right.$
$\Leftrightarrow$ $\left[\begin{matrix} x = 0 + 5\\ x = 0 + 1\end{matrix}\right.$
$\Leftrightarrow$ $\left[\begin{matrix} x = 5\\ x = 1\end{matrix}\right.$
$\text{Vậy}$ `x` $\in$ `{5 ; 1}`
`c,`
`x^{2} - 2x = 24`
$\Leftrightarrow$ ` x^{2} - 2x - 24 = 0`
$\Leftrightarrow$ ` x^{2} - 6x + 4x - 24 = 0`
$\Leftrightarrow$ ` x(x-6) + 4(x-6) = 0`
$\Leftrightarrow$ ` (x-6)(x+4) = 0`
$\Leftrightarrow$ $\left[\begin{matrix} x - 6 = 0\\ x + 4 = 0\end{matrix}\right.$
$\Leftrightarrow$ $\left[\begin{matrix} x = 0 + 6\\ x = 0 - 4\end{matrix}\right.$
$\Leftrightarrow$ $\left[\begin{matrix} x = 6\\ x = -4\end{matrix}\right.$
$\text{Vậy}$ `x` $\in$ `{6 ; -4}`
