Given the inequality:
$$- 2 \cos{\left(\frac{x}{3} \right)} < \sqrt{3}$$
To solve this inequality, we must first solve the corresponding equation:
$$- 2 \cos{\left(\frac{x}{3} \right)} = \sqrt{3}$$
Solve:
Given the equation
$$- 2 \cos{\left(\frac{x}{3} \right)} = \sqrt{3}$$
- this is the simplest trigonometric equation
Divide both parts of the equation by -2
The equation is transformed to
$$\cos{\left(\frac{x}{3} \right)} = - \frac{\sqrt{3}}{2}$$
This equation is transformed to
$$\frac{x}{3} = \pi n + \operatorname{acos}{\left(- \frac{\sqrt{3}}{2} \right)}$$
$$\frac{x}{3} = \pi n - \pi + \operatorname{acos}{\left(- \frac{\sqrt{3}}{2} \right)}$$
Or
$$\frac{x}{3} = \pi n + \frac{5 \pi}{6}$$
$$\frac{x}{3} = \pi n - \frac{\pi}{6}$$
, where n - is a integer
Divide both parts of the equation by
$$\frac{1}{3}$$
$$x_{1} = 3 \pi n + \frac{5 \pi}{2}$$
$$x_{2} = 3 \pi n - \frac{\pi}{2}$$
$$x_{1} = 3 \pi n + \frac{5 \pi}{2}$$
$$x_{2} = 3 \pi n - \frac{\pi}{2}$$
This roots
$$x_{1} = 3 \pi n + \frac{5 \pi}{2}$$
$$x_{2} = 3 \pi n - \frac{\pi}{2}$$
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
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$\left(3 \pi n + \frac{5 \pi}{2}\right) + - \frac{1}{10}$$
=
$$3 \pi n - \frac{1}{10} + \frac{5 \pi}{2}$$
substitute to the expression
$$- 2 \cos{\left(\frac{x}{3} \right)} < \sqrt{3}$$
$$- 2 \cos{\left(\frac{3 \pi n - \frac{1}{10} + \frac{5 \pi}{2}}{3} \right)} < \sqrt{3}$$
/ 1 pi \ ___
2*sin|- -- + -- + pi*n| < \/ 3
\ 30 3 /
one of the solutions of our inequality is:
$$x < 3 \pi n + \frac{5 \pi}{2}$$
_____ _____
\ /
-------ο-------ο-------
x1 x2
Other solutions will get with the changeover to the next point
etc.
The answer:
$$x < 3 \pi n + \frac{5 \pi}{2}$$
$$x > 3 \pi n - \frac{\pi}{2}$$