Đáp án:
$\begin{array}{l}
3)a)\sqrt[3]{x} = \dfrac{{ - 2}}{3}\\
\Leftrightarrow x = {\left( {\dfrac{{ - 2}}{3}} \right)^3}\\
\Leftrightarrow x = \dfrac{{ - 8}}{{27}}\\
Vậy\,x = \dfrac{{ - 8}}{{27}}\\
b)\sqrt[3]{{3 - x}} = 0,2\\
\Leftrightarrow 3 - x = {\left( {0,2} \right)^3}\\
\Leftrightarrow 3 - x = 0,008\\
\Leftrightarrow x = 2,992\\
Vậy\,x = 2,992\\
B4)\\
a)\sqrt[3]{{2x}} + \sqrt[3]{{54x}} - \sqrt[3]{{ - 250x}}\\
= \sqrt[3]{{2x}} + 3\sqrt[3]{{2x}} + 5\sqrt[3]{{2x}}\\
= 9\sqrt[3]{{2x}}\\
b)\sqrt[3]{{8x - 16}} + \sqrt[3]{{x - 2}} - \dfrac{2}{3}\sqrt[3]{{27x - 54}}\\
= \sqrt[3]{{8\left( {x - 2} \right)}} + \sqrt[3]{{x - 2}} - \dfrac{2}{3}.\sqrt[3]{{27\left( {x - 2} \right)}}\\
= 2\sqrt[3]{{x - 2}} + \sqrt[3]{{x - 2}} - \dfrac{2}{3}.3\sqrt[3]{{x - 2}}\\
= \sqrt[3]{{x - 2}}
\end{array}$
