Se da la desigualdad:
$$\left(\left(\sqrt{15} + 4\right)^{2 x} - 8 \left(\frac{1}{\sqrt{15} + 4}\right)^{x}\right) + 1 \geq 0$$
Para resolver esta desigualdad primero hay que resolver la ecuación correspondiente:
$$\left(\left(\sqrt{15} + 4\right)^{2 x} - 8 \left(\frac{1}{\sqrt{15} + 4}\right)^{x}\right) + 1 = 0$$
Resolvemos:
Tenemos la ecuación:
$$\left(\left(\sqrt{15} + 4\right)^{2 x} - 8 \left(\frac{1}{\sqrt{15} + 4}\right)^{x}\right) + 1 = 0$$
o
$$\left(\left(\sqrt{15} + 4\right)^{2 x} - 8 \left(\frac{1}{\sqrt{15} + 4}\right)^{x}\right) + 1 = 0$$
Sustituimos
$$v = \left(\sqrt{15} + 4\right)^{2 x}$$
obtendremos
$$v + 1 - 8 \left(\sqrt{15} + 4\right)^{- x} = 0$$
o
$$v + 1 - 8 \left(\sqrt{15} + 4\right)^{- x} = 0$$
hacemos cambio inverso
$$\left(\sqrt{15} + 4\right)^{2 x} = v$$
o
$$x = \frac{\log{\left(v \right)}}{\log{\left(\left(\sqrt{15} + 4\right)^{2} \right)}}$$
$$x_{1} = \frac{- \frac{\log{\left(36 + \sqrt{1299} \right)}}{3} - \frac{2 \log{\left(3 \right)}}{3} + \log{\left(- \sqrt[3]{3} + \left(36 + \sqrt{1299}\right)^{\frac{2}{3}} \right)}}{\log{\left(\sqrt{15} + 4 \right)}}$$
$$x_{2} = \frac{\log{\left(\frac{\sqrt[3]{3} \left(4 \sqrt[3]{3} - \left(1 - \sqrt{3} i\right)^{2} \left(36 + \sqrt{1299}\right)^{\frac{2}{3}}\right)}{6 \left(1 - \sqrt{3} i\right) \sqrt[3]{36 + \sqrt{1299}}} \right)}}{\log{\left(\sqrt{15} + 4 \right)}}$$
$$x_{3} = \frac{\log{\left(\frac{\sqrt[3]{3} \left(4 \sqrt[3]{3} - \left(1 + \sqrt{3} i\right)^{2} \left(36 + \sqrt{1299}\right)^{\frac{2}{3}}\right)}{6 \left(1 + \sqrt{3} i\right) \sqrt[3]{36 + \sqrt{1299}}} \right)}}{\log{\left(\sqrt{15} + 4 \right)}}$$
Descartamos las soluciones complejas:
$$x_{1} = \frac{- \frac{\log{\left(36 + \sqrt{1299} \right)}}{3} - \frac{2 \log{\left(3 \right)}}{3} + \log{\left(- \sqrt[3]{3} + \left(36 + \sqrt{1299}\right)^{\frac{2}{3}} \right)}}{\log{\left(\sqrt{15} + 4 \right)}}$$
Las raíces dadas
$$x_{1} = \frac{- \frac{\log{\left(36 + \sqrt{1299} \right)}}{3} - \frac{2 \log{\left(3 \right)}}{3} + \log{\left(- \sqrt[3]{3} + \left(36 + \sqrt{1299}\right)^{\frac{2}{3}} \right)}}{\log{\left(\sqrt{15} + 4 \right)}}$$
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
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$- \frac{1}{10} + \frac{- \frac{\log{\left(36 + \sqrt{1299} \right)}}{3} - \frac{2 \log{\left(3 \right)}}{3} + \log{\left(- \sqrt[3]{3} + \left(36 + \sqrt{1299}\right)^{\frac{2}{3}} \right)}}{\log{\left(\sqrt{15} + 4 \right)}}$$
=
$$- \frac{1}{10} + \frac{- \frac{\log{\left(36 + \sqrt{1299} \right)}}{3} - \frac{2 \log{\left(3 \right)}}{3} + \log{\left(- \sqrt[3]{3} + \left(36 + \sqrt{1299}\right)^{\frac{2}{3}} \right)}}{\log{\left(\sqrt{15} + 4 \right)}}$$
lo sustituimos en la expresión
$$\left(\left(\sqrt{15} + 4\right)^{2 x} - 8 \left(\frac{1}{\sqrt{15} + 4}\right)^{x}\right) + 1 \geq 0$$
$$\left(- 8 \left(\frac{1}{\sqrt{15} + 4}\right)^{- \frac{1}{10} + \frac{- \frac{\log{\left(36 + \sqrt{1299} \right)}}{3} - \frac{2 \log{\left(3 \right)}}{3} + \log{\left(- \sqrt[3]{3} + \left(36 + \sqrt{1299}\right)^{\frac{2}{3}} \right)}}{\log{\left(\sqrt{15} + 4 \right)}}} + \left(\sqrt{15} + 4\right)^{2 \left(- \frac{1}{10} + \frac{- \frac{\log{\left(36 + \sqrt{1299} \right)}}{3} - \frac{2 \log{\left(3 \right)}}{3} + \log{\left(- \sqrt[3]{3} + \left(36 + \sqrt{1299}\right)^{\frac{2}{3}} \right)}}{\log{\left(\sqrt{15} + 4 \right)}}\right)}\right) + 1 \geq 0$$
/ / ______\ / 2/3 \\ / ______\ / 2/3 \
| 2*log(3) log\36 + \/ 1299 / |/ ______\ 3 ___|| 2*log(3) log\36 + \/ 1299 / |/ ______\ 3 ___|
2*|- -------- - ------------------ + log\\36 + \/ 1299 / - \/ 3 /| - -------- - ------------------ + log\\36 + \/ 1299 / - \/ 3 /
1 \ 3 3 / 1 3 3
- - + --------------------------------------------------------------------- -- - ----------------------------------------------------------------- >= 0
5 / ____\ 10 / ____\
log\4 + \/ 15 / log\4 + \/ 15 /
/ ____\ / ____\
1 + \4 + \/ 15 / - 8*\4 + \/ 15 /
pero
/ / ______\ / 2/3 \\ / ______\ / 2/3 \
| 2*log(3) log\36 + \/ 1299 / |/ ______\ 3 ___|| 2*log(3) log\36 + \/ 1299 / |/ ______\ 3 ___|
2*|- -------- - ------------------ + log\\36 + \/ 1299 / - \/ 3 /| - -------- - ------------------ + log\\36 + \/ 1299 / - \/ 3 /
1 \ 3 3 / 1 3 3
- - + --------------------------------------------------------------------- -- - ----------------------------------------------------------------- < 0
5 / ____\ 10 / ____\
log\4 + \/ 15 / log\4 + \/ 15 /
/ ____\ / ____\
1 + \4 + \/ 15 / - 8*\4 + \/ 15 /
Entonces
$$x \leq \frac{- \frac{\log{\left(36 + \sqrt{1299} \right)}}{3} - \frac{2 \log{\left(3 \right)}}{3} + \log{\left(- \sqrt[3]{3} + \left(36 + \sqrt{1299}\right)^{\frac{2}{3}} \right)}}{\log{\left(\sqrt{15} + 4 \right)}}$$
no se cumple
significa que la solución de la desigualdad será con:
$$x \geq \frac{- \frac{\log{\left(36 + \sqrt{1299} \right)}}{3} - \frac{2 \log{\left(3 \right)}}{3} + \log{\left(- \sqrt[3]{3} + \left(36 + \sqrt{1299}\right)^{\frac{2}{3}} \right)}}{\log{\left(\sqrt{15} + 4 \right)}}$$
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