5) Ta có:
$(x - 3)^2 \geq 0, \forall x$
$(x + y - 5)^2 \geq 0, \forall x,y$
$\Rightarrow (x - 3)^2 + (x + y - 5)^2 \geq 0$
Dấu "=" xảy ra $\Leftrightarrow \begin{cases}x - 3 = 0\\x + y - 5 = 0 \end{cases}$
$\Leftrightarrow \begin{cases} x = 3\\y = 2\end{cases}$
6) $\dfrac{x +4}{2} = \dfrac{2x + 1}{3}$
$\Leftrightarrow 3(x + 4) = 2(2x + 1)$
$\Leftrightarrow 3x + 12 = 4x + 2$
$\Leftrightarrow x = 10$
7) $\dfrac{3(x + 2)^2}{5} = \dfrac{75}{x +2}$
$\Leftrightarrow (x+2)^2(x+2) = \dfrac{75.5}{3}$
$\Leftrightarrow (x + 2)^3 = 125$
$\Leftrightarrow x + 2 = \sqrt[3]{125} = 5$
$\Leftrightarrow x = 3$
