Se da la desigualdad:
$$- \frac{\left(x + 2\right)^{2}}{2 x - 5} + \left(x^{2} - 2 x\right) < 0$$
Para resolver esta desigualdad primero hay que resolver la ecuación correspondiente:
$$- \frac{\left(x + 2\right)^{2}}{2 x - 5} + \left(x^{2} - 2 x\right) = 0$$
Resolvemos:
$$x_{1} = \frac{5}{3} + \left(- \frac{1}{2} - \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}} + \frac{16}{9 \left(- \frac{1}{2} - \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}}$$
$$x_{2} = \frac{5}{3} + \frac{16}{9 \left(- \frac{1}{2} + \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}} + \left(- \frac{1}{2} + \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}$$
$$x_{3} = \frac{16}{9 \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}} + \frac{5}{3} + \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}$$
Descartamos las soluciones complejas:
$$x_{1} = \frac{16}{9 \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}} + \frac{5}{3} + \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}$$
Las raíces dadas
$$x_{1} = \frac{16}{9 \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}} + \frac{5}{3} + \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}$$
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
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$- \frac{1}{10} + \left(\frac{16}{9 \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}} + \frac{5}{3} + \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}\right)$$
=
$$\frac{16}{9 \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}} + \frac{47}{30} + \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}$$
lo sustituimos en la expresión
$$- \frac{\left(x + 2\right)^{2}}{2 x - 5} + \left(x^{2} - 2 x\right) < 0$$
$$- \frac{\left(2 + \left(\frac{16}{9 \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}} + \frac{47}{30} + \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}\right)\right)^{2}}{-5 + 2 \left(\frac{16}{9 \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}} + \frac{47}{30} + \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}\right)} + \left(- 2 \left(\frac{16}{9 \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}} + \frac{47}{30} + \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}\right) + \left(\frac{16}{9 \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}} + \frac{47}{30} + \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}\right)^{2}\right) < 0$$
2
/ ________________ \
| / ______ |
|107 / 169 \/ 1353 16 |
|--- + 3 / --- + -------- + -----------------------|
| 30 \/ 54 18 ________________|
2 | / ______ |
/ ________________ \ ________________ | / 169 \/ 1353 |
| / ______ | / ______ | 9*3 / --- + -------- |
47 |47 / 169 \/ 1353 16 | / 169 \/ 1353 32 \ \/ 54 18 /
- -- + |-- + 3 / --- + -------- + -----------------------| - 2*3 / --- + -------- - ----------------------- - -------------------------------------------------------- < 0
15 |30 \/ 54 18 ________________| \/ 54 18 ________________ ________________
| / ______ | / ______ / ______
| / 169 \/ 1353 | / 169 \/ 1353 28 / 169 \/ 1353 32
| 9*3 / --- + -------- | 9*3 / --- + -------- - -- + 2*3 / --- + -------- + -----------------------
\ \/ 54 18 / \/ 54 18 15 \/ 54 18 ________________
/ ______
/ 169 \/ 1353
9*3 / --- + --------
\/ 54 18
significa que la solución de la desigualdad será con:
$$x < \frac{16}{9 \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}} + \frac{5}{3} + \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}$$
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