Se da la desigualdad:
$$-6 + \frac{x^{2} \left(x + 2\right) \left(x - 1\right)}{x} < 0$$
Para resolver esta desigualdad primero hay que resolver la ecuación correspondiente:
$$-6 + \frac{x^{2} \left(x + 2\right) \left(x - 1\right)}{x} = 0$$
Resolvemos:
$$x_{1} = - \frac{1}{3} + \left(- \frac{1}{2} - \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}} + \frac{7}{9 \left(- \frac{1}{2} - \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}}$$
$$x_{2} = - \frac{1}{3} + \frac{7}{9 \left(- \frac{1}{2} + \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}} + \left(- \frac{1}{2} + \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}$$
$$x_{3} = - \frac{1}{3} + \frac{7}{9 \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}} + \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}$$
Descartamos las soluciones complejas:
$$x_{1} = - \frac{1}{3} + \frac{7}{9 \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}} + \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}$$
Las raíces dadas
$$x_{1} = - \frac{1}{3} + \frac{7}{9 \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}} + \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}$$
son puntos de cambio del signo de desigualdad en las soluciones.
Primero definámonos con el signo hasta el punto extremo izquierdo:
$$x_{0} < x_{1}$$
Consideremos, por ejemplo, el punto
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$- \frac{1}{10} + \left(- \frac{1}{3} + \frac{7}{9 \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}} + \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}\right)$$
=
$$- \frac{13}{30} + \frac{7}{9 \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}} + \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}$$
lo sustituimos en la expresión
$$-6 + \frac{x^{2} \left(x + 2\right) \left(x - 1\right)}{x} < 0$$
$$-6 + \frac{\left(\left(- \frac{13}{30} + \frac{7}{9 \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}} + \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}\right) + 2\right) \left(- \frac{13}{30} + \frac{7}{9 \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}} + \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}\right)^{2} \left(-1 + \left(- \frac{13}{30} + \frac{7}{9 \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}} + \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}\right)\right)}{- \frac{13}{30} + \frac{7}{9 \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}} + \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}} < 0$$
/ _____________ \ / _____________ \ / _____________ \
| / ____ | | / ____ | | / ____ |
| 43 / 71 \/ 58 7 | | 13 / 71 \/ 58 7 | |47 / 71 \/ 58 7 |
-6 + |- -- + 3 / -- + ------ + --------------------|*|- -- + 3 / -- + ------ + --------------------|*|-- + 3 / -- + ------ + --------------------|
| 30 \/ 27 3 _____________| | 30 \/ 27 3 _____________| |30 \/ 27 3 _____________| < 0
| / ____ | | / ____ | | / ____ |
| / 71 \/ 58 | | / 71 \/ 58 | | / 71 \/ 58 |
| 9*3 / -- + ------ | | 9*3 / -- + ------ | | 9*3 / -- + ------ |
\ \/ 27 3 / \ \/ 27 3 / \ \/ 27 3 /
significa que la solución de la desigualdad será con:
$$x < - \frac{1}{3} + \frac{7}{9 \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}} + \sqrt[3]{\frac{\sqrt{58}}{3} + \frac{71}{27}}$$
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