2 câu trả lời
#andy
\[\begin{array}{l}
x - \sqrt {5x + 4} = 2\\
DKXD:x \ge 2\\
\Leftrightarrow \sqrt {5x + 4} = x - 2\\
\Leftrightarrow 5x + 4 = {(x - 2)^2}\\
\Leftrightarrow 5x + 4 = {x^2} - 4x + 4\\
\Leftrightarrow {x^2} - 4x - 5x + 4 - 4 = 0\\
\Leftrightarrow {x^2} - 9x = 0\\
\Leftrightarrow x(x - 9) = 0\\
\Leftrightarrow \left[ \begin{array}{l}
x = 0\\
x - 9 = 0
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
x = 0\left( {KTMDK} \right)\\
x = 9\left( {TMDK} \right)
\end{array} \right.\\
\Rightarrow x = 9
\end{array}\]
${x-\sqrt[]{5x+4}=2}$ ${ĐKXĐ:x≥2}$
${⇔\sqrt[]{5x+4}=x-2}$
${⇔5x+4=(x-2)²}$
${⇔5x+4=x²-4x+4}$
${⇔x²-4x-5x+4-4=0}$
${⇔x²-9x=0}$
${⇔x(x-9)=0}$
${⇔\left[\begin{matrix} x=0\\ x-9=0\end{matrix}\right.}$
${⇔\left[\begin{matrix} x=0,ktm\\ x=9,tm\end{matrix}\right.}$
$\text{Vậy x = 9}$