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(x+1)*(x^2-1)>sqrt(2)/2 desigualdades

En la desigualdad la incógnita

Solución

Ha introducido [src]
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$$\left(x + 1\right) \left(x^{2} - 1\right) > \frac{\sqrt{2}}{2}$$
(x + 1)*(x^2 - 1) > sqrt(2)/2
Solución detallada
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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|     /                ________________________________________________                                                              \ | /              ________________________________________________                                                              \        
|     |               /              _________________________                                                                       | | |             /              _________________________                                                                       |        
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|     |  13      /      8       /        64   \  27     2  /     \/ 2                                  4                             | | |17      /      8       /        64   \  27     2  /     \/ 2                                  4                             |     ___
|-1 + |- -- + 3 /       -- +   /      - --- + ---------------  + -----  + -----------------------------------------------------------| |*|-- + 3 /       -- +   /      - --- + ---------------  + -----  + -----------------------------------------------------------|   \/ 2 
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|     |                                                                   9*3 /       -- +   /      - --- + ---------------  + ----- | | |                                                                 9*3 /       -- +   /      - --- + ---------------  + ----- |        
\     \                                                                     \/        27   \/         729          4             4   / / \                                                                   \/        27   \/         729          4             4   /        
        

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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Solución de la desigualdad en el gráfico
Respuesta rápida [src]
   /                         _________________________________________________                                                               \
   |                        /          ___________________________                                                                           |
   |                       /          /                         2                                                                            |
   |              3 ___ 3 /          /          /           ___\          ___                                2/3                             |
   |          1   \/ 2 *\/    32 + \/   -1024 + \-32 - 27*\/ 2 /   + 27*\/ 2                              4*2                                |
And|x < oo, - - + ------------------------------------------------------------ + -------------------------------------------------------- < x|
   |          3                                6                                        _________________________________________________    |
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   |                                                                                  /          /                         2                 |
   |                                                                               3 /          /          /           ___\          ___     |
   \                                                                             3*\/    32 + \/   -1024 + \-32 - 27*\/ 2 /   + 27*\/ 2      /
$$x < \infty \wedge - \frac{1}{3} + \frac{4 \cdot 2^{\frac{2}{3}}}{3 \sqrt[3]{32 + 27 \sqrt{2} + \sqrt{-1024 + \left(- 27 \sqrt{2} - 32\right)^{2}}}} + \frac{\sqrt[3]{2} \sqrt[3]{32 + 27 \sqrt{2} + \sqrt{-1024 + \left(- 27 \sqrt{2} - 32\right)^{2}}}}{6} < x$$
(x < oo)∧(-1/3 + 2^(1/3)*(32 + sqrt(-1024 + (-32 - 27*sqrt(2))^2) + 27*sqrt(2))^(1/3)/6 + 4*2^(2/3)/(3*(32 + sqrt(-1024 + (-32 - 27*sqrt(2))^2) + 27*sqrt(2))^(1/3)) < x)
Respuesta rápida 2 [src]
                  _________________________________________________                                                                
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                /          /                         2                                                                             
       3 ___ 3 /          /          /           ___\          ___                                2/3                              
   1   \/ 2 *\/    32 + \/   -1024 + \-32 - 27*\/ 2 /   + 27*\/ 2                              4*2                                 
(- - + ------------------------------------------------------------ + --------------------------------------------------------, oo)
   3                                6                                        _________________________________________________     
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                                                                        3 /          /          /           ___\          ___      
                                                                      3*\/    32 + \/   -1024 + \-32 - 27*\/ 2 /   + 27*\/ 2       
$$x\ in\ \left(- \frac{1}{3} + \frac{4 \cdot 2^{\frac{2}{3}}}{3 \sqrt[3]{32 + 27 \sqrt{2} + \sqrt{-1024 + \left(- 27 \sqrt{2} - 32\right)^{2}}}} + \frac{\sqrt[3]{2} \sqrt[3]{32 + 27 \sqrt{2} + \sqrt{-1024 + \left(- 27 \sqrt{2} - 32\right)^{2}}}}{6}, \infty\right)$$
x in Interval.open(-1/3 + 4*2^(2/3)/(3*(32 + 27*sqrt(2) + sqrt(-1024 + (-27*sqrt(2) - 32)^2))^(1/3)) + 2^(1/3)*(32 + 27*sqrt(2) + sqrt(-1024 + (-27*sqrt(2) - 32)^2))^(1/3)/6, oo)