Se da la desigualdad:
$$- \left(x - 3\right) \left(x + 2\right) + \left(\left(x^{2} + 5 x\right) + 7\right)^{2} < 1$$
Para resolver esta desigualdad primero hay que resolver la ecuación correspondiente:
$$- \left(x - 3\right) \left(x + 2\right) + \left(\left(x^{2} + 5 x\right) + 7\right)^{2} = 1$$
Resolvemos:
$$x_{1} = -2$$
$$x_{2} = - \frac{8}{3} + \frac{2}{3 \left(- \frac{1}{2} - \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}} - \frac{\left(- \frac{1}{2} - \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}{3}$$
$$x_{3} = - \frac{8}{3} - \frac{\left(- \frac{1}{2} + \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}{3} + \frac{2}{3 \left(- \frac{1}{2} + \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}$$
$$x_{4} = - \frac{8}{3} - \frac{\sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}{3} + \frac{2}{3 \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}$$
Descartamos las soluciones complejas:
$$x_{1} = -2$$
$$x_{2} = - \frac{8}{3} - \frac{\sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}{3} + \frac{2}{3 \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}$$
Las raíces dadas
$$x_{2} = - \frac{8}{3} - \frac{\sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}{3} + \frac{2}{3 \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}$$
$$x_{1} = -2$$
son puntos de cambio del signo de desigualdad en las soluciones.
Primero definámonos con el signo hasta el punto extremo izquierdo:
$$x_{0} < x_{2}$$
Consideremos, por ejemplo, el punto
$$x_{0} = x_{2} - \frac{1}{10}$$
=
$$\left(- \frac{8}{3} - \frac{\sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}{3} + \frac{2}{3 \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}\right) + - \frac{1}{10}$$
=
$$- \frac{83}{30} - \frac{\sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}{3} + \frac{2}{3 \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}$$
lo sustituimos en la expresión
$$- \left(x - 3\right) \left(x + 2\right) + \left(\left(x^{2} + 5 x\right) + 7\right)^{2} < 1$$
$$- \left(\left(- \frac{83}{30} - \frac{\sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}{3} + \frac{2}{3 \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}\right) - 3\right) \left(\left(- \frac{83}{30} - \frac{\sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}{3} + \frac{2}{3 \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}\right) + 2\right) + \left(\left(5 \left(- \frac{83}{30} - \frac{\sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}{3} + \frac{2}{3 \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}\right) + \left(- \frac{83}{30} - \frac{\sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}{3} + \frac{2}{3 \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}\right)^{2}\right) + 7\right)^{2} < 1$$
2
/ 2 \
| / _________________ \ _________________ | / _________________ \ / _________________ \
| | / _____ | / _____ | | / _____ | | / _____ |
| | / 169 9*\/ 353 | / 169 9*\/ 353 | | / 169 9*\/ 353 | | / 169 9*\/ 353 |
| | 3 / --- + --------- | 5*3 / --- + --------- | | 3 / --- + --------- | | 3 / --- + --------- |
| 41 | 83 \/ 2 2 2 | \/ 2 2 10 | | 173 \/ 2 2 2 | | 23 \/ 2 2 2 | < 1
|- -- + |- -- - ---------------------- + ------------------------| - ------------------------ + ------------------------| - |- --- - ---------------------- + ------------------------|*|- -- - ---------------------- + ------------------------|
| 6 | 30 3 _________________| 3 _________________| | 30 3 _________________| | 30 3 _________________|
| | / _____ | / _____ | | / _____ | | / _____ |
| | / 169 9*\/ 353 | / 169 9*\/ 353 | | / 169 9*\/ 353 | | / 169 9*\/ 353 |
| | 3*3 / --- + --------- | 3*3 / --- + --------- | | 3*3 / --- + --------- | | 3*3 / --- + --------- |
\ \ \/ 2 2 / \/ 2 2 / \ \/ 2 2 / \ \/ 2 2 /
pero
2
/ 2 \
| / _________________ \ _________________ | / _________________ \ / _________________ \
| | / _____ | / _____ | | / _____ | | / _____ |
| | / 169 9*\/ 353 | / 169 9*\/ 353 | | / 169 9*\/ 353 | | / 169 9*\/ 353 |
| | 3 / --- + --------- | 5*3 / --- + --------- | | 3 / --- + --------- | | 3 / --- + --------- |
| 41 | 83 \/ 2 2 2 | \/ 2 2 10 | | 173 \/ 2 2 2 | | 23 \/ 2 2 2 | > 1
|- -- + |- -- - ---------------------- + ------------------------| - ------------------------ + ------------------------| - |- --- - ---------------------- + ------------------------|*|- -- - ---------------------- + ------------------------|
| 6 | 30 3 _________________| 3 _________________| | 30 3 _________________| | 30 3 _________________|
| | / _____ | / _____ | | / _____ | | / _____ |
| | / 169 9*\/ 353 | / 169 9*\/ 353 | | / 169 9*\/ 353 | | / 169 9*\/ 353 |
| | 3*3 / --- + --------- | 3*3 / --- + --------- | | 3*3 / --- + --------- | | 3*3 / --- + --------- |
\ \ \/ 2 2 / \/ 2 2 / \ \/ 2 2 / \ \/ 2 2 /
Entonces
$$x < - \frac{8}{3} - \frac{\sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}{3} + \frac{2}{3 \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}$$
no se cumple
significa que una de las soluciones de nuestra ecuación será con:
$$x > - \frac{8}{3} - \frac{\sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}}{3} + \frac{2}{3 \sqrt[3]{\frac{169}{2} + \frac{9 \sqrt{353}}{2}}} \wedge x < -2$$
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/ \
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x2 x1