Se da la desigualdad:
$$\left(x + 1\right) \left(x^{2} - 1\right) > \frac{\sqrt{2}}{2}$$
Para resolver esta desigualdad primero hay que resolver la ecuación correspondiente:
$$\left(x + 1\right) \left(x^{2} - 1\right) = \frac{\sqrt{2}}{2}$$
Resolvemos:
$$x_{1} = - \frac{1}{3} + \left(- \frac{1}{2} - \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}} + \frac{4}{9 \left(- \frac{1}{2} - \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}}$$
$$x_{2} = - \frac{1}{3} + \frac{4}{9 \left(- \frac{1}{2} + \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}} + \left(- \frac{1}{2} + \frac{\sqrt{3} i}{2}\right) \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}$$
$$x_{3} = - \frac{1}{3} + \frac{4}{9 \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}} + \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}$$
Descartamos las soluciones complejas:
$$x_{1} = - \frac{1}{3} + \frac{4}{9 \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}} + \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}$$
Las raíces dadas
$$x_{1} = - \frac{1}{3} + \frac{4}{9 \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}} + \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}$$
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
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$- \frac{1}{10} + \left(- \frac{1}{3} + \frac{4}{9 \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}} + \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}\right)$$
=
$$- \frac{13}{30} + \frac{4}{9 \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}} + \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}$$
lo sustituimos en la expresión
$$\left(x + 1\right) \left(x^{2} - 1\right) > \frac{\sqrt{2}}{2}$$
$$\left(-1 + \left(- \frac{13}{30} + \frac{4}{9 \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}} + \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}\right)^{2}\right) \left(1 + \left(- \frac{13}{30} + \frac{4}{9 \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}} + \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}\right)\right) > \frac{\sqrt{2}}{2}$$
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Entonces
$$x < - \frac{1}{3} + \frac{4}{9 \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}} + \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}$$
no se cumple
significa que la solución de la desigualdad será con:
$$x > - \frac{1}{3} + \frac{4}{9 \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}} + \sqrt[3]{\frac{8}{27} + \frac{\sqrt{2}}{4} + \sqrt{- \frac{64}{729} + \frac{\left(- \frac{\sqrt{2}}{2} - \frac{16}{27}\right)^{2}}{4}}}$$
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