Se da la desigualdad:
$$\frac{\left(8 x + \left(10 x^{3} + 81 x^{2}\right)\right) + 8}{x + 8} \geq 0$$
Para resolver esta desigualdad primero hay que resolver la ecuación correspondiente:
$$\frac{\left(8 x + \left(10 x^{3} + 81 x^{2}\right)\right) + 8}{x + 8} = 0$$
Resolvemos:
$$x_{1} = - \frac{27}{10} - \frac{2107}{100 \left(- \frac{1}{2} - \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}} - \frac{\left(- \frac{1}{2} - \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}{3}$$
$$x_{2} = - \frac{27}{10} - \frac{\left(- \frac{1}{2} + \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}{3} - \frac{2107}{100 \left(- \frac{1}{2} + \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}$$
$$x_{3} = - \frac{\sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}{3} - \frac{27}{10} - \frac{2107}{100 \sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}$$
Descartamos las soluciones complejas:
$$x_{1} = - \frac{\sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}{3} - \frac{27}{10} - \frac{2107}{100 \sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}$$
Las raíces dadas
$$x_{1} = - \frac{\sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}{3} - \frac{27}{10} - \frac{2107}{100 \sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}$$
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
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$\left(- \frac{\sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}{3} - \frac{27}{10} - \frac{2107}{100 \sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}\right) + - \frac{1}{10}$$
=
$$- \frac{\sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}{3} - \frac{14}{5} - \frac{2107}{100 \sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}$$
lo sustituimos en la expresión
$$\frac{\left(8 x + \left(10 x^{3} + 81 x^{2}\right)\right) + 8}{x + 8} \geq 0$$
$$\frac{\left(8 \left(- \frac{\sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}{3} - \frac{14}{5} - \frac{2107}{100 \sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}\right) + \left(10 \left(- \frac{\sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}{3} - \frac{14}{5} - \frac{2107}{100 \sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}\right)^{3} + 81 \left(- \frac{\sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}{3} - \frac{14}{5} - \frac{2107}{100 \sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}\right)^{2}\right)\right) + 8}{\left(- \frac{\sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}{3} - \frac{14}{5} - \frac{2107}{100 \sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}\right) + 8} \geq 0$$
3 2
/ ________________________\ / ________________________\ ________________________
| / _________ | | / _________ | / _________
| / 513081 3*\/ 2971194 | | / 513081 3*\/ 2971194 | / 513081 3*\/ 2971194
| 3 / ------ + ------------- | | 3 / ------ + ------------- | 8*3 / ------ + -------------
72 | 14 2107 \/ 1000 50 | | 14 2107 \/ 1000 50 | 4214 \/ 1000 50
- -- + 10*|- -- - --------------------------------- - -----------------------------| + 81*|- -- - --------------------------------- - -----------------------------| - -------------------------------- - -------------------------------
5 | 5 ________________________ 3 | | 5 ________________________ 3 | ________________________ 3
| / _________ | | / _________ | / _________
| / 513081 3*\/ 2971194 | | / 513081 3*\/ 2971194 | / 513081 3*\/ 2971194
| 100*3 / ------ + ------------- | | 100*3 / ------ + ------------- | 25*3 / ------ + -------------
\ \/ 1000 50 / \ \/ 1000 50 / \/ 1000 50
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- >= 0
________________________
/ _________
/ 513081 3*\/ 2971194
3 / ------ + -------------
26 2107 \/ 1000 50
-- - --------------------------------- - -----------------------------
5 ________________________ 3
/ _________
/ 513081 3*\/ 2971194
100*3 / ------ + -------------
\/ 1000 50
significa que la solución de la desigualdad será con:
$$x \leq - \frac{\sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}{3} - \frac{27}{10} - \frac{2107}{100 \sqrt[3]{\frac{3 \sqrt{2971194}}{50} + \frac{513081}{1000}}}$$
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