Se da la desigualdad:
$$-3 + \frac{\left(2 - 5 x\right) \left(x + 2\right)^{2}}{x} \leq 0$$
Para resolver esta desigualdad primero hay que resolver la ecuación correspondiente:
$$-3 + \frac{\left(2 - 5 x\right) \left(x + 2\right)^{2}}{x} = 0$$
Resolvemos:
$$x_{1} = - \frac{6}{5} + \left(- \frac{1}{2} - \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}} + \frac{11}{25 \left(- \frac{1}{2} - \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}}$$
$$x_{2} = - \frac{6}{5} + \frac{11}{25 \left(- \frac{1}{2} + \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}} + \left(- \frac{1}{2} + \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}$$
$$x_{3} = - \frac{6}{5} + \frac{11}{25 \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}} + \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}$$
Descartamos las soluciones complejas:
$$x_{1} = - \frac{6}{5} + \frac{11}{25 \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}} + \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}$$
Las raíces dadas
$$x_{1} = - \frac{6}{5} + \frac{11}{25 \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}} + \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}$$
son puntos de cambio del signo de desigualdad en las soluciones.
Primero definámonos con el signo hasta el punto extremo izquierdo:
$$x_{0} \leq x_{1}$$
Consideremos, por ejemplo, el punto
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$- \frac{1}{10} + \left(- \frac{6}{5} + \frac{11}{25 \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}} + \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}\right)$$
=
$$- \frac{13}{10} + \frac{11}{25 \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}} + \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}$$
lo sustituimos en la expresión
$$-3 + \frac{\left(2 - 5 x\right) \left(x + 2\right)^{2}}{x} \leq 0$$
$$-3 + \frac{\left(2 - 5 \left(- \frac{13}{10} + \frac{11}{25 \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}} + \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}\right)\right) \left(\left(- \frac{13}{10} + \frac{11}{25 \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}} + \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}\right) + 2\right)^{2}}{- \frac{13}{10} + \frac{11}{25 \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}} + \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}} \leq 0$$
2
/ _______________ \ / _______________ \
| / _____ | | / _____ |
|7 / 109 \/ 422 11 | |17 / 109 \/ 422 11 |
|-- + 3 / --- + ------- + -----------------------| *|-- - 5*3 / --- + ------- - ----------------------|
|10 \/ 125 25 _______________| |2 \/ 125 25 _______________|
| / _____ | | / _____ |
| / 109 \/ 422 | | / 109 \/ 422 |
| 25*3 / --- + ------- | | 5*3 / --- + ------- |
\ \/ 125 25 / \ \/ 125 25 /
-3 + ------------------------------------------------------------------------------------------------------------- <= 0
_______________
/ _____
13 / 109 \/ 422 11
- -- + 3 / --- + ------- + -----------------------
10 \/ 125 25 _______________
/ _____
/ 109 \/ 422
25*3 / --- + -------
\/ 125 25
pero
2
/ _______________ \ / _______________ \
| / _____ | | / _____ |
|7 / 109 \/ 422 11 | |17 / 109 \/ 422 11 |
|-- + 3 / --- + ------- + -----------------------| *|-- - 5*3 / --- + ------- - ----------------------|
|10 \/ 125 25 _______________| |2 \/ 125 25 _______________|
| / _____ | | / _____ |
| / 109 \/ 422 | | / 109 \/ 422 |
| 25*3 / --- + ------- | | 5*3 / --- + ------- |
\ \/ 125 25 / \ \/ 125 25 /
-3 + ------------------------------------------------------------------------------------------------------------- >= 0
_______________
/ _____
13 / 109 \/ 422 11
- -- + 3 / --- + ------- + -----------------------
10 \/ 125 25 _______________
/ _____
/ 109 \/ 422
25*3 / --- + -------
\/ 125 25
Entonces
$$x \leq - \frac{6}{5} + \frac{11}{25 \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}} + \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}$$
no se cumple
significa que la solución de la desigualdad será con:
$$x \geq - \frac{6}{5} + \frac{11}{25 \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}} + \sqrt[3]{\frac{\sqrt{422}}{25} + \frac{109}{125}}$$
_____
/
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