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4*sin(x)*cos(x)>=(2)^(1/2) desigualdades

En la desigualdad la incógnita

Solución

Ha introducido [src]
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4*sin(x)*cos(x) >= \/ 2 
$$4 \sin{\left(x \right)} \cos{\left(x \right)} \geq \sqrt{2}$$
(4*sin(x))*cos(x) >= sqrt(2)
Solución detallada
Se da la desigualdad:
$$4 \sin{\left(x \right)} \cos{\left(x \right)} \geq \sqrt{2}$$
Para resolver esta desigualdad primero hay que resolver la ecuación correspondiente:
$$4 \sin{\left(x \right)} \cos{\left(x \right)} = \sqrt{2}$$
Resolvemos:
$$x_{1} = - 2 \operatorname{atan}{\left(-1 + \sqrt{2} \sqrt{2 - \sqrt{2}} + \sqrt{2} \right)}$$
$$x_{2} = - 2 \operatorname{atan}{\left(1 + \sqrt{2} + \sqrt{2} \sqrt{\sqrt{2} + 2} \right)}$$
$$x_{3} = 2 \operatorname{atan}{\left(- \sqrt{2} + 1 + \sqrt{2} \sqrt{2 - \sqrt{2}} \right)}$$
$$x_{4} = - 2 \operatorname{atan}{\left(- \sqrt{2} \sqrt{\sqrt{2} + 2} + 1 + \sqrt{2} \right)}$$
$$x_{1} = - 2 \operatorname{atan}{\left(-1 + \sqrt{2} \sqrt{2 - \sqrt{2}} + \sqrt{2} \right)}$$
$$x_{2} = - 2 \operatorname{atan}{\left(1 + \sqrt{2} + \sqrt{2} \sqrt{\sqrt{2} + 2} \right)}$$
$$x_{3} = 2 \operatorname{atan}{\left(- \sqrt{2} + 1 + \sqrt{2} \sqrt{2 - \sqrt{2}} \right)}$$
$$x_{4} = - 2 \operatorname{atan}{\left(- \sqrt{2} \sqrt{\sqrt{2} + 2} + 1 + \sqrt{2} \right)}$$
Las raíces dadas
$$x_{2} = - 2 \operatorname{atan}{\left(1 + \sqrt{2} + \sqrt{2} \sqrt{\sqrt{2} + 2} \right)}$$
$$x_{1} = - 2 \operatorname{atan}{\left(-1 + \sqrt{2} \sqrt{2 - \sqrt{2}} + \sqrt{2} \right)}$$
$$x_{4} = - 2 \operatorname{atan}{\left(- \sqrt{2} \sqrt{\sqrt{2} + 2} + 1 + \sqrt{2} \right)}$$
$$x_{3} = 2 \operatorname{atan}{\left(- \sqrt{2} + 1 + \sqrt{2} \sqrt{2 - \sqrt{2}} \right)}$$
son puntos de cambio del signo de desigualdad en las soluciones.
Primero definámonos con el signo hasta el punto extremo izquierdo:
$$x_{0} \leq x_{2}$$
Consideremos, por ejemplo, el punto
$$x_{0} = x_{2} - \frac{1}{10}$$
=
$$- 2 \operatorname{atan}{\left(1 + \sqrt{2} + \sqrt{2} \sqrt{\sqrt{2} + 2} \right)} - \frac{1}{10}$$
=
$$- 2 \operatorname{atan}{\left(1 + \sqrt{2} + \sqrt{2} \sqrt{\sqrt{2} + 2} \right)} - \frac{1}{10}$$
lo sustituimos en la expresión
$$4 \sin{\left(x \right)} \cos{\left(x \right)} \geq \sqrt{2}$$
$$4 \sin{\left(- 2 \operatorname{atan}{\left(1 + \sqrt{2} + \sqrt{2} \sqrt{\sqrt{2} + 2} \right)} - \frac{1}{10} \right)} \cos{\left(- 2 \operatorname{atan}{\left(1 + \sqrt{2} + \sqrt{2} \sqrt{\sqrt{2} + 2} \right)} - \frac{1}{10} \right)} \geq \sqrt{2}$$
      /           /                     ___________\\    /           /                     ___________\\         
      |1          |      ___     ___   /       ___ ||    |1          |      ___     ___   /       ___ ||      ___
-4*cos|-- + 2*atan\1 + \/ 2  + \/ 2 *\/  2 + \/ 2  /|*sin|-- + 2*atan\1 + \/ 2  + \/ 2 *\/  2 + \/ 2  /| >= \/ 2 
      \10                                           /    \10                                           /    
         

pero
      /           /                     ___________\\    /           /                     ___________\\        
      |1          |      ___     ___   /       ___ ||    |1          |      ___     ___   /       ___ ||     ___
-4*cos|-- + 2*atan\1 + \/ 2  + \/ 2 *\/  2 + \/ 2  /|*sin|-- + 2*atan\1 + \/ 2  + \/ 2 *\/  2 + \/ 2  /| < \/ 2 
      \10                                           /    \10                                           /   
        

Entonces
$$x \leq - 2 \operatorname{atan}{\left(1 + \sqrt{2} + \sqrt{2} \sqrt{\sqrt{2} + 2} \right)}$$
no se cumple
significa que una de las soluciones de nuestra ecuación será con:
$$x \geq - 2 \operatorname{atan}{\left(1 + \sqrt{2} + \sqrt{2} \sqrt{\sqrt{2} + 2} \right)} \wedge x \leq - 2 \operatorname{atan}{\left(-1 + \sqrt{2} \sqrt{2 - \sqrt{2}} + \sqrt{2} \right)}$$
         _____           _____  
        /     \         /     \  
-------•-------•-------•-------•-------
       x2      x1      x4      x3

Recibiremos otras soluciones de la desigualdad pasando al polo siguiente etc.
etc.
Respuesta:
$$x \geq - 2 \operatorname{atan}{\left(1 + \sqrt{2} + \sqrt{2} \sqrt{\sqrt{2} + 2} \right)} \wedge x \leq - 2 \operatorname{atan}{\left(-1 + \sqrt{2} \sqrt{2 - \sqrt{2}} + \sqrt{2} \right)}$$
$$x \geq - 2 \operatorname{atan}{\left(- \sqrt{2} \sqrt{\sqrt{2} + 2} + 1 + \sqrt{2} \right)} \wedge x \leq 2 \operatorname{atan}{\left(- \sqrt{2} + 1 + \sqrt{2} \sqrt{2 - \sqrt{2}} \right)}$$
Solución de la desigualdad en el gráfico
Respuesta rápida [src]
   /         /   ___________\      /   ___________\     \
   |         |  /       ___ |      |  /       ___ |     |
   |         |\/  2 + \/ 2  |      |\/  2 - \/ 2  |     |
And|x <= atan|--------------|, atan|--------------| <= x|
   |         |   ___________|      |   ___________|     |
   |         |  /       ___ |      |  /       ___ |     |
   \         \\/  2 - \/ 2  /      \\/  2 + \/ 2  /     /
$$x \leq \operatorname{atan}{\left(\frac{\sqrt{\sqrt{2} + 2}}{\sqrt{2 - \sqrt{2}}} \right)} \wedge \operatorname{atan}{\left(\frac{\sqrt{2 - \sqrt{2}}}{\sqrt{\sqrt{2} + 2}} \right)} \leq x$$
(x <= atan(sqrt(2 + sqrt(2))/sqrt(2 - sqrt(2))))∧(atan(sqrt(2 - sqrt(2))/sqrt(2 + sqrt(2))) <= x)
Respuesta rápida 2 [src]
     /   ___________\      /   ___________\ 
     |  /       ___ |      |  /       ___ | 
     |\/  2 - \/ 2  |      |\/  2 + \/ 2  | 
[atan|--------------|, atan|--------------|]
     |   ___________|      |   ___________| 
     |  /       ___ |      |  /       ___ | 
     \\/  2 + \/ 2  /      \\/  2 - \/ 2  / 
$$x\ in\ \left[\operatorname{atan}{\left(\frac{\sqrt{2 - \sqrt{2}}}{\sqrt{\sqrt{2} + 2}} \right)}, \operatorname{atan}{\left(\frac{\sqrt{\sqrt{2} + 2}}{\sqrt{2 - \sqrt{2}}} \right)}\right]$$
x in Interval(atan(sqrt(2 - sqrt(2))/sqrt(sqrt(2) + 2)), atan(sqrt(sqrt(2) + 2)/sqrt(2 - sqrt(2))))