Given the inequality:
$$\sin{\left(x \right)} \tan{\left(2 x \right)} > 0$$
To solve this inequality, we must first solve the corresponding equation:
$$\sin{\left(x \right)} \tan{\left(2 x \right)} = 0$$
Solve:
Given the equation:
$$\sin{\left(x \right)} \tan{\left(2 x \right)} = 0$$
Transform
$$\sin{\left(x \right)} \tan{\left(2 x \right)} = 0$$
$$\sin{\left(x \right)} \tan{\left(2 x \right)} = 0$$
Consider each factor separately
Step
$$\sin{\left(x \right)} = 0$$
- this is the simplest trigonometric equation
We get:
$$\sin{\left(x \right)} = 0$$
This equation is transformed to
$$x = 2 \pi n + \operatorname{asin}{\left(0 \right)}$$
$$x = 2 \pi n - \operatorname{asin}{\left(0 \right)} + \pi$$
Or
$$x = 2 \pi n$$
$$x = 2 \pi n + \pi$$
, where n - is a integer
Step
$$\tan{\left(2 x \right)} = 0$$
- this is the simplest trigonometric equation
We get:
$$\tan{\left(2 x \right)} = 0$$
This equation is transformed to
$$2 x = \pi n + \operatorname{atan}{\left(0 \right)}$$
Or
$$2 x = \pi n$$
, where n - is a integer
Divide both parts of the equation by
$$2$$
get the intermediate answer:
$$x = 2 \pi n$$
$$x = 2 \pi n + \pi$$
$$x = \frac{\pi n}{2}$$
The final answer:
$$x_{1} = 2 \pi n$$
$$x_{2} = 2 \pi n + \pi$$
$$x_{3} = \frac{\pi n}{2}$$
$$x_{1} = 2 \pi n$$
$$x_{2} = 2 \pi n + \pi$$
$$x_{3} = \frac{\pi n}{2}$$
$$x_{1} = 2 \pi n$$
$$x_{2} = 2 \pi n + \pi$$
$$x_{3} = \frac{\pi n}{2}$$
This roots
$$x_{1} = 2 \pi n$$
$$x_{2} = 2 \pi n + \pi$$
$$x_{3} = \frac{\pi n}{2}$$
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
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$2 \pi n - \frac{1}{10}$$
=
$$2 \pi n - \frac{1}{10}$$
substitute to the expression
$$\sin{\left(x \right)} \tan{\left(2 x \right)} > 0$$
$$\sin{\left(2 \pi n - \frac{1}{10} \right)} \tan{\left(2 \cdot \left(2 \pi n - \frac{1}{10}\right) \right)} > 0$$
sin(1/10)*tan(1/5) > 0
one of the solutions of our inequality is:
$$x < 2 \pi n$$
_____ _____
\ / \
-------ο-------ο-------ο-------
x_1 x_2 x_3
Other solutions will get with the changeover to the next point
etc.
The answer:
$$x < 2 \pi n$$
$$x > 2 \pi n + \pi \wedge x < \frac{\pi n}{2}$$