Se da la desigualdad:
− ( x + 2 ) 2 2 x − 5 + ( x 2 − 2 x ) < 0 - \frac{\left(x + 2\right)^{2}}{2 x - 5} + \left(x^{2} - 2 x\right) < 0 − 2 x − 5 ( x + 2 ) 2 + ( x 2 − 2 x ) < 0 Para resolver esta desigualdad primero hay que resolver la ecuación correspondiente:
− ( x + 2 ) 2 2 x − 5 + ( x 2 − 2 x ) = 0 - \frac{\left(x + 2\right)^{2}}{2 x - 5} + \left(x^{2} - 2 x\right) = 0 − 2 x − 5 ( x + 2 ) 2 + ( x 2 − 2 x ) = 0 Resolvemos:
x 1 = 5 3 + ( − 1 2 − 3 i 2 ) 1353 18 + 169 54 3 + 16 9 ( − 1 2 − 3 i 2 ) 1353 18 + 169 54 3 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 1 = 3 5 + ( − 2 1 − 2 3 i ) 3 18 1353 + 54 169 + 9 ( − 2 1 − 2 3 i ) 3 18 1353 + 54 169 16 x 2 = 5 3 + 16 9 ( − 1 2 + 3 i 2 ) 1353 18 + 169 54 3 + ( − 1 2 + 3 i 2 ) 1353 18 + 169 54 3 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 2 = 3 5 + 9 ( − 2 1 + 2 3 i ) 3 18 1353 + 54 169 16 + ( − 2 1 + 2 3 i ) 3 18 1353 + 54 169 x 3 = 16 9 1353 18 + 169 54 3 + 5 3 + 1353 18 + 169 54 3 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}} x 3 = 9 3 18 1353 + 54 169 16 + 3 5 + 3 18 1353 + 54 169 Descartamos las soluciones complejas:
x 1 = 16 9 1353 18 + 169 54 3 + 5 3 + 1353 18 + 169 54 3 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}} x 1 = 9 3 18 1353 + 54 169 16 + 3 5 + 3 18 1353 + 54 169 Las raíces dadas
x 1 = 16 9 1353 18 + 169 54 3 + 5 3 + 1353 18 + 169 54 3 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}} x 1 = 9 3 18 1353 + 54 169 16 + 3 5 + 3 18 1353 + 54 169 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 x_{0} < x_{1} x 0 < x 1 Consideremos, por ejemplo, el punto
x 0 = x 1 − 1 10 x_{0} = x_{1} - \frac{1}{10} x 0 = x 1 − 10 1 =
− 1 10 + ( 16 9 1353 18 + 169 54 3 + 5 3 + 1353 18 + 169 54 3 ) - \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) − 10 1 + 9 3 18 1353 + 54 169 16 + 3 5 + 3 18 1353 + 54 169 =
16 9 1353 18 + 169 54 3 + 47 30 + 1353 18 + 169 54 3 \frac{16}{9 \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}}} + \frac{47}{30} + \sqrt[3]{\frac{\sqrt{1353}}{18} + \frac{169}{54}} 9 3 18 1353 + 54 169 16 + 30 47 + 3 18 1353 + 54 169 lo sustituimos en la expresión
− ( x + 2 ) 2 2 x − 5 + ( x 2 − 2 x ) < 0 - \frac{\left(x + 2\right)^{2}}{2 x - 5} + \left(x^{2} - 2 x\right) < 0 − 2 x − 5 ( x + 2 ) 2 + ( x 2 − 2 x ) < 0 − ( 2 + ( 16 9 1353 18 + 169 54 3 + 47 30 + 1353 18 + 169 54 3 ) ) 2 − 5 + 2 ( 16 9 1353 18 + 169 54 3 + 47 30 + 1353 18 + 169 54 3 ) + ( − 2 ( 16 9 1353 18 + 169 54 3 + 47 30 + 1353 18 + 169 54 3 ) + ( 16 9 1353 18 + 169 54 3 + 47 30 + 1353 18 + 169 54 3 ) 2 ) < 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 − − 5 + 2 ( 9 3 18 1353 + 54 169 16 + 30 47 + 3 18 1353 + 54 169 ) ( 2 + ( 9 3 18 1353 + 54 169 16 + 30 47 + 3 18 1353 + 54 169 ) ) 2 + − 2 9 3 18 1353 + 54 169 16 + 30 47 + 3 18 1353 + 54 169 + 9 3 18 1353 + 54 169 16 + 30 47 + 3 18 1353 + 54 169 2 < 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 < 16 9 1353 18 + 169 54 3 + 5 3 + 1353 18 + 169 54 3 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}} x < 9 3 18 1353 + 54 169 16 + 3 5 + 3 18 1353 + 54 169 _____
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x1