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cos(2x)^2-sin(2x)^2<=sqrt(3)/2
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cos(2x)^2-sin(2x)^2<=sqrt(3)/2 inequation

A inequation with variable

The solution

You have entered [src]
                           ___
   2           2         \/ 3 
cos (2*x) - sin (2*x) <= -----
                           2  
$$- \sin^{2}{\left(2 x \right)} + \cos^{2}{\left(2 x \right)} \leq \frac{\sqrt{3}}{2}$$
-sin(2*x)^2 + cos(2*x)^2 <= sqrt(3)/2
Detail solution
Given the inequality:
$$- \sin^{2}{\left(2 x \right)} + \cos^{2}{\left(2 x \right)} \leq \frac{\sqrt{3}}{2}$$
To solve this inequality, we must first solve the corresponding equation:
$$- \sin^{2}{\left(2 x \right)} + \cos^{2}{\left(2 x \right)} = \frac{\sqrt{3}}{2}$$
Solve:
Given the equation
$$- \sin^{2}{\left(2 x \right)} + \cos^{2}{\left(2 x \right)} = \frac{\sqrt{3}}{2}$$
transform
$$2 \cos^{2}{\left(2 x \right)} - 1 - \frac{\sqrt{3}}{2} = 0$$
$$2 \cos^{2}{\left(2 x \right)} - 1 - \frac{\sqrt{3}}{2} = 0$$
Do replacement
$$w = \cos{\left(2 x \right)}$$
This equation is of the form
$$a\ w^2 + b\ w + c = 0$$
A quadratic equation can be solved using the discriminant
The roots of the quadratic equation:
$$w_{1} = \frac{\sqrt{D} - b}{2 a}$$
$$w_{2} = \frac{- \sqrt{D} - b}{2 a}$$
where $D = b^2 - 4 a c$ is the discriminant.
Because
$$a = 2$$
$$b = 0$$
$$c = -1 - \frac{\sqrt{3}}{2}$$
, then
$$D = b^2 - 4\ a\ c = $$
$$0^{2} - 2 \cdot 4 \left(-1 - \frac{\sqrt{3}}{2}\right) = 4 \sqrt{3} + 8$$
Because D > 0, then the equation has two roots.
$$w_1 = \frac{(-b + \sqrt{D})}{2 a}$$
$$w_2 = \frac{(-b - \sqrt{D})}{2 a}$$
or
$$w_{1} = \frac{\sqrt{4 \sqrt{3} + 8}}{4}$$
Simplify
$$w_{2} = - \frac{\sqrt{4 \sqrt{3} + 8}}{4}$$
Simplify
do backward replacement
$$\cos{\left(2 x \right)} = w$$
$$\cos{\left(2 x \right)} = w$$
- this is the simplest trigonometric equation
This equation is transformed to
$$2 x = 2 \pi n + \operatorname{acos}{\left(w \right)}$$
$$2 x = 2 \pi n + \operatorname{acos}{\left(w \right)} - \pi$$
Or
$$2 x = 2 \pi n + \operatorname{acos}{\left(w \right)}$$
$$2 x = 2 \pi n + \operatorname{acos}{\left(w \right)} - \pi$$
, where n - is a integer
Divide both parts of the equation by
$$2$$
get the intermediate answer:
$$x = \pi n + \frac{\operatorname{acos}{\left(w \right)}}{2}$$
$$x = \pi n + \frac{\operatorname{acos}{\left(w \right)}}{2} - \frac{\pi}{2}$$
substitute w:
$$x_{1} = \pi n + \frac{\operatorname{acos}{\left(w_{1} \right)}}{2}$$
$$x_{1} = \pi n + \frac{\operatorname{acos}{\left(\frac{\sqrt{4 \sqrt{3} + 8}}{4} \right)}}{2}$$
$$x_{1} = \pi n + \frac{\operatorname{acos}{\left(\frac{\sqrt{4 \sqrt{3} + 8}}{4} \right)}}{2}$$
$$x_{2} = \pi n + \frac{\operatorname{acos}{\left(w_{2} \right)}}{2}$$
$$x_{2} = \pi n + \frac{\operatorname{acos}{\left(- \frac{\sqrt{4 \sqrt{3} + 8}}{4} \right)}}{2}$$
$$x_{2} = \pi n + \frac{\operatorname{acos}{\left(- \frac{\sqrt{4 \sqrt{3} + 8}}{4} \right)}}{2}$$
$$x_{3} = \pi n + \frac{\operatorname{acos}{\left(w_{1} \right)}}{2} - \frac{\pi}{2}$$
$$x_{3} = \pi n - \frac{\pi}{2} + \frac{\operatorname{acos}{\left(\frac{\sqrt{4 \sqrt{3} + 8}}{4} \right)}}{2}$$
$$x_{3} = \pi n - \frac{\pi}{2} + \frac{\operatorname{acos}{\left(\frac{\sqrt{4 \sqrt{3} + 8}}{4} \right)}}{2}$$
$$x_{4} = \pi n + \frac{\operatorname{acos}{\left(w_{2} \right)}}{2} - \frac{\pi}{2}$$
$$x_{4} = \pi n - \frac{\pi}{2} + \frac{\operatorname{acos}{\left(- \frac{\sqrt{4 \sqrt{3} + 8}}{4} \right)}}{2}$$
$$x_{4} = \pi n - \frac{\pi}{2} + \frac{\operatorname{acos}{\left(- \frac{\sqrt{4 \sqrt{3} + 8}}{4} \right)}}{2}$$
$$x_{1} = - \operatorname{atan}{\left(\frac{\sqrt{\sqrt{3} + 6 + 4 \sqrt{\sqrt{3} + 2}}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
$$x_{2} = \operatorname{atan}{\left(\frac{\sqrt{\sqrt{3} + 6 + 4 \sqrt{\sqrt{3} + 2}}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
$$x_{3} = - \operatorname{atan}{\left(\frac{\sqrt{- 4 \sqrt{\sqrt{3} + 2} + \sqrt{3} + 6}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
$$x_{4} = \operatorname{atan}{\left(\frac{\sqrt{- 4 \sqrt{\sqrt{3} + 2} + \sqrt{3} + 6}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
$$x_{1} = - \operatorname{atan}{\left(\frac{\sqrt{\sqrt{3} + 6 + 4 \sqrt{\sqrt{3} + 2}}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
$$x_{2} = \operatorname{atan}{\left(\frac{\sqrt{\sqrt{3} + 6 + 4 \sqrt{\sqrt{3} + 2}}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
$$x_{3} = - \operatorname{atan}{\left(\frac{\sqrt{- 4 \sqrt{\sqrt{3} + 2} + \sqrt{3} + 6}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
$$x_{4} = \operatorname{atan}{\left(\frac{\sqrt{- 4 \sqrt{\sqrt{3} + 2} + \sqrt{3} + 6}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
This roots
$$x_{1} = - \operatorname{atan}{\left(\frac{\sqrt{\sqrt{3} + 6 + 4 \sqrt{\sqrt{3} + 2}}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
$$x_{3} = - \operatorname{atan}{\left(\frac{\sqrt{- 4 \sqrt{\sqrt{3} + 2} + \sqrt{3} + 6}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
$$x_{4} = \operatorname{atan}{\left(\frac{\sqrt{- 4 \sqrt{\sqrt{3} + 2} + \sqrt{3} + 6}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
$$x_{2} = \operatorname{atan}{\left(\frac{\sqrt{\sqrt{3} + 6 + 4 \sqrt{\sqrt{3} + 2}}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
is the points with change the sign of the inequality expression.
First define with the sign to the leftmost point:
$$x_{0} \leq x_{1}$$
For example, let's take the point
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$- \operatorname{atan}{\left(\frac{\sqrt{\sqrt{3} + 6 + 4 \sqrt{\sqrt{3} + 2}}}{\sqrt{- \sqrt{3} + 2}} \right)} - \frac{1}{10}$$
=
$$- \operatorname{atan}{\left(\frac{\sqrt{\sqrt{3} + 6 + 4 \sqrt{\sqrt{3} + 2}}}{\sqrt{- \sqrt{3} + 2}} \right)} - \frac{1}{10}$$
substitute to the expression
$$- \sin^{2}{\left(2 x \right)} + \cos^{2}{\left(2 x \right)} \leq \frac{\sqrt{3}}{2}$$
$$- \sin^{2}{\left(2 \left(- \operatorname{atan}{\left(\frac{\sqrt{\sqrt{3} + 6 + 4 \sqrt{\sqrt{3} + 2}}}{\sqrt{- \sqrt{3} + 2}} \right)} - \frac{1}{10}\right) \right)} + \cos^{2}{\left(2 \left(- \operatorname{atan}{\left(\frac{\sqrt{\sqrt{3} + 6 + 4 \sqrt{\sqrt{3} + 2}}}{\sqrt{- \sqrt{3} + 2}} \right)} - \frac{1}{10}\right) \right)} \leq \frac{\sqrt{3}}{2}$$
    /          /    ______________________________\\       /          /    ______________________________\\         
    |          |   /                  ___________ ||       |          |   /                  ___________ ||         
    |          |  /        ___       /       ___  ||       |          |  /        ___       /       ___  ||      ___
   2|1         |\/   6 + \/ 3  + 4*\/  2 + \/ 3   ||      2|1         |\/   6 + \/ 3  + 4*\/  2 + \/ 3   ||    \/ 3 
cos |- + 2*atan|----------------------------------|| - sin |- + 2*atan|----------------------------------|| <= -----
    |5         |             ___________          ||       |5         |             ___________          ||      2  
    |          |            /       ___           ||       |          |            /       ___           ||    
    \          \          \/  2 - \/ 3            //       \          \          \/  2 - \/ 3            //         
         

but
    /          /    ______________________________\\       /          /    ______________________________\\         
    |          |   /                  ___________ ||       |          |   /                  ___________ ||         
    |          |  /        ___       /       ___  ||       |          |  /        ___       /       ___  ||      ___
   2|1         |\/   6 + \/ 3  + 4*\/  2 + \/ 3   ||      2|1         |\/   6 + \/ 3  + 4*\/  2 + \/ 3   ||    \/ 3 
cos |- + 2*atan|----------------------------------|| - sin |- + 2*atan|----------------------------------|| >= -----
    |5         |             ___________          ||       |5         |             ___________          ||      2  
    |          |            /       ___           ||       |          |            /       ___           ||    
    \          \          \/  2 - \/ 3            //       \          \          \/  2 - \/ 3            //         
         

Then
$$x \leq - \operatorname{atan}{\left(\frac{\sqrt{\sqrt{3} + 6 + 4 \sqrt{\sqrt{3} + 2}}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
no execute
one of the solutions of our inequality is:
$$x \geq - \operatorname{atan}{\left(\frac{\sqrt{\sqrt{3} + 6 + 4 \sqrt{\sqrt{3} + 2}}}{\sqrt{- \sqrt{3} + 2}} \right)} \wedge x \leq - \operatorname{atan}{\left(\frac{\sqrt{- 4 \sqrt{\sqrt{3} + 2} + \sqrt{3} + 6}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
         _____           _____  
        /     \         /     \  
-------•-------•-------•-------•-------
       x_1      x_3      x_4      x_2

Other solutions will get with the changeover to the next point
etc.
The answer:
$$x \geq - \operatorname{atan}{\left(\frac{\sqrt{\sqrt{3} + 6 + 4 \sqrt{\sqrt{3} + 2}}}{\sqrt{- \sqrt{3} + 2}} \right)} \wedge x \leq - \operatorname{atan}{\left(\frac{\sqrt{- 4 \sqrt{\sqrt{3} + 2} + \sqrt{3} + 6}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
$$x \geq \operatorname{atan}{\left(\frac{\sqrt{- 4 \sqrt{\sqrt{3} + 2} + \sqrt{3} + 6}}{\sqrt{- \sqrt{3} + 2}} \right)} \wedge x \leq \operatorname{atan}{\left(\frac{\sqrt{\sqrt{3} + 6 + 4 \sqrt{\sqrt{3} + 2}}}{\sqrt{- \sqrt{3} + 2}} \right)}$$
Solving inequality on a graph
Rapid solution [src]
  /   /              /  ___     ___\           /  ___     ___\     \     /              /  ___     ___\       /  ___     ___\      \\
  |   |              |\/ 2  - \/ 6 |           |\/ 2  - \/ 6 |     |     |              |\/ 2  - \/ 6 |       |\/ 2  - \/ 6 |      ||
  |   |          atan|-------------|       atan|-------------|     |     |          atan|-------------|  -atan|-------------|      ||
  |   |              |  ___     ___|           |  ___     ___|     |     |              |  ___     ___|       |  ___     ___|      ||
  |   |              \\/ 2  + \/ 6 /  pi       \\/ 2  + \/ 6 /     |     |     pi       \\/ 2  + \/ 6 /       \\/ 2  + \/ 6 /      ||
Or|And|x <= pi + -------------------, -- - ------------------- <= x|, And|x <= -- + -------------------, --------------------- <= x||
  \   \                   2           2             2              /     \     2             2                     2               //
$$\left(x \leq \frac{\operatorname{atan}{\left(\frac{- \sqrt{6} + \sqrt{2}}{\sqrt{2} + \sqrt{6}} \right)}}{2} + \pi \wedge - \frac{\operatorname{atan}{\left(\frac{- \sqrt{6} + \sqrt{2}}{\sqrt{2} + \sqrt{6}} \right)}}{2} + \frac{\pi}{2} \leq x\right) \vee \left(x \leq \frac{\operatorname{atan}{\left(\frac{- \sqrt{6} + \sqrt{2}}{\sqrt{2} + \sqrt{6}} \right)}}{2} + \frac{\pi}{2} \wedge - \frac{\operatorname{atan}{\left(\frac{- \sqrt{6} + \sqrt{2}}{\sqrt{2} + \sqrt{6}} \right)}}{2} \leq x\right)$$
((-atan((sqrt(2) - sqrt(6))/(sqrt(2) + sqrt(6)))/2 <= x)∧(x <= pi/2 + atan((sqrt(2) - sqrt(6))/(sqrt(2) + sqrt(6)))/2))∨((x <= pi + atan((sqrt(2) - sqrt(6))/(sqrt(2) + sqrt(6)))/2)∧(pi/2 - atan((sqrt(2) - sqrt(6))/(sqrt(2) + sqrt(6)))/2 <= x))
Rapid solution 2 [src]
      /  ___     ___\            /  ___     ___\              /  ___     ___\           /  ___     ___\ 
      |\/ 2  - \/ 6 |            |\/ 2  - \/ 6 |              |\/ 2  - \/ 6 |           |\/ 2  - \/ 6 | 
 -atan|-------------|        atan|-------------|          atan|-------------|       atan|-------------| 
      |  ___     ___|            |  ___     ___|              |  ___     ___|           |  ___     ___| 
      \\/ 2  + \/ 6 /   pi       \\/ 2  + \/ 6 /     pi       \\/ 2  + \/ 6 /           \\/ 2  + \/ 6 / 
[---------------------, -- + -------------------] U [-- - -------------------, pi + -------------------]
           2            2             2              2             2                         2          
$$x\ in\ \left[- \frac{\operatorname{atan}{\left(\frac{- \sqrt{6} + \sqrt{2}}{\sqrt{2} + \sqrt{6}} \right)}}{2}, \frac{\operatorname{atan}{\left(\frac{- \sqrt{6} + \sqrt{2}}{\sqrt{2} + \sqrt{6}} \right)}}{2} + \frac{\pi}{2}\right] \cup \left[- \frac{\operatorname{atan}{\left(\frac{- \sqrt{6} + \sqrt{2}}{\sqrt{2} + \sqrt{6}} \right)}}{2} + \frac{\pi}{2}, \frac{\operatorname{atan}{\left(\frac{- \sqrt{6} + \sqrt{2}}{\sqrt{2} + \sqrt{6}} \right)}}{2} + \pi\right]$$
x in Union(Interval(-atan((-sqrt(6) + sqrt(2))/(sqrt(2) + sqrt(6)))/2 + pi/2, atan((-sqrt(6) + sqrt(2))/(sqrt(2) + sqrt(6)))/2 + pi), Interval(-atan((-sqrt(6) + sqrt(2))/(sqrt(2) + sqrt(6)))/2, atan((-sqrt(6) + sqrt(2))/(sqrt(2) + sqrt(6)))/2 + pi/2))
The graph
cos(2x)^2-sin(2x)^2<=sqrt(3)/2 inequation