Given the inequality:
$$\sin{\left(2 x \right)} \geq - \frac{\sqrt{2}}{2}$$
To solve this inequality, we must first solve the corresponding equation:
$$\sin{\left(2 x \right)} = - \frac{\sqrt{2}}{2}$$
Solve:
Given the equation
$$\sin{\left(2 x \right)} = - \frac{\sqrt{2}}{2}$$
- this is the simplest trigonometric equation
This equation is transformed to
$$2 x = 2 \pi n + \operatorname{asin}{\left(- \frac{\sqrt{2}}{2} \right)}$$
$$2 x = 2 \pi n - \operatorname{asin}{\left(- \frac{\sqrt{2}}{2} \right)} + \pi$$
Or
$$2 x = 2 \pi n - \frac{\pi}{4}$$
$$2 x = 2 \pi n + \frac{5 \pi}{4}$$
, where n - is a integer
Divide both parts of the equation by
$$2$$
$$x_{1} = \pi n - \frac{\pi}{8}$$
$$x_{2} = \pi n + \frac{5 \pi}{8}$$
$$x_{1} = \pi n - \frac{\pi}{8}$$
$$x_{2} = \pi n + \frac{5 \pi}{8}$$
This roots
$$x_{1} = \pi n - \frac{\pi}{8}$$
$$x_{2} = \pi n + \frac{5 \pi}{8}$$
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
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$\left(\pi n - \frac{\pi}{8}\right) + - \frac{1}{10}$$
=
$$\pi n - \frac{\pi}{8} - \frac{1}{10}$$
substitute to the expression
$$\sin{\left(2 x \right)} \geq - \frac{\sqrt{2}}{2}$$
$$\sin{\left(2 \left(\pi n - \frac{\pi}{8} - \frac{1}{10}\right) \right)} \geq - \frac{\sqrt{2}}{2}$$
___
/1 pi \ -\/ 2
-sin|- + -- - 2*pi*n| >= -------
\5 4 / 2
but
___
/1 pi \ -\/ 2
-sin|- + -- - 2*pi*n| < -------
\5 4 / 2
Then
$$x \leq \pi n - \frac{\pi}{8}$$
no execute
one of the solutions of our inequality is:
$$x \geq \pi n - \frac{\pi}{8} \wedge x \leq \pi n + \frac{5 \pi}{8}$$
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/ \
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x1 x2