4x^3+9x=0.........................

`4x^3 + 9x = 0`

`⇔ x(4x^2 + 9) = 0`

$\ ⇔ \left[ \begin{array}{l}x=0\\4x^2+9=0\end{array} \right.$

$\ ⇔ \left[ \begin{array}{l}x=0\\4x^2=-9\end{array} \right.$

$\ ⇔ \left[ \begin{array}{l}x=0\\x^2=\dfrac{-9}{4} \text{(vô lí)}\end{array} \right.$

Vậy `x = 0`