Given the inequality:
$$\cos{\left(6 x \right)} < - \frac{\sqrt{3}}{2}$$
To solve this inequality, we must first solve the corresponding equation:
$$\cos{\left(6 x \right)} = - \frac{\sqrt{3}}{2}$$
Solve:
Given the equation
$$\cos{\left(6 x \right)} = - \frac{\sqrt{3}}{2}$$
- this is the simplest trigonometric equation
This equation is transformed to
$$6 x = \pi n + \operatorname{acos}{\left(- \frac{\sqrt{3}}{2} \right)}$$
$$6 x = \pi n - \pi + \operatorname{acos}{\left(- \frac{\sqrt{3}}{2} \right)}$$
Or
$$6 x = \pi n + \frac{5 \pi}{6}$$
$$6 x = \pi n - \frac{\pi}{6}$$
, where n - is a integer
Divide both parts of the equation by
$$6$$
$$x_{1} = \frac{\pi n}{6} + \frac{5 \pi}{36}$$
$$x_{2} = \frac{\pi n}{6} - \frac{\pi}{36}$$
$$x_{1} = \frac{\pi n}{6} + \frac{5 \pi}{36}$$
$$x_{2} = \frac{\pi n}{6} - \frac{\pi}{36}$$
This roots
$$x_{1} = \frac{\pi n}{6} + \frac{5 \pi}{36}$$
$$x_{2} = \frac{\pi n}{6} - \frac{\pi}{36}$$
is the points with change the sign of the inequality expression.
First define with the sign to the leftmost point:
$$x_{0} < x_{1}$$
For example, let's take the point
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$\left(\frac{\pi n}{6} + \frac{5 \pi}{36}\right) + - \frac{1}{10}$$
=
$$\frac{\pi n}{6} - \frac{1}{10} + \frac{5 \pi}{36}$$
substitute to the expression
$$\cos{\left(6 x \right)} < - \frac{\sqrt{3}}{2}$$
$$\cos{\left(6 \left(\frac{\pi n}{6} - \frac{1}{10} + \frac{5 \pi}{36}\right) \right)} < - \frac{\sqrt{3}}{2}$$
___
/ 3 pi \ -\/ 3
-sin|- - + -- + pi*n| < -------
\ 5 3 / 2
but
___
/ 3 pi \ -\/ 3
-sin|- - + -- + pi*n| > -------
\ 5 3 / 2
Then
$$x < \frac{\pi n}{6} + \frac{5 \pi}{36}$$
no execute
one of the solutions of our inequality is:
$$x > \frac{\pi n}{6} + \frac{5 \pi}{36} \wedge x < \frac{\pi n}{6} - \frac{\pi}{36}$$
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/ \
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x1 x2