Given the inequality:
$$\sin{\left(\frac{x}{7} \right)} > - \frac{1}{5}$$
To solve this inequality, we must first solve the corresponding equation:
$$\sin{\left(\frac{x}{7} \right)} = - \frac{1}{5}$$
Solve:
Given the equation
$$\sin{\left(\frac{x}{7} \right)} = - \frac{1}{5}$$
- this is the simplest trigonometric equation
This equation is transformed to
$$\frac{x}{7} = 2 \pi n + \operatorname{asin}{\left(- \frac{1}{5} \right)}$$
$$\frac{x}{7} = 2 \pi n - \operatorname{asin}{\left(- \frac{1}{5} \right)} + \pi$$
Or
$$\frac{x}{7} = 2 \pi n - \operatorname{asin}{\left(\frac{1}{5} \right)}$$
$$\frac{x}{7} = 2 \pi n + \operatorname{asin}{\left(\frac{1}{5} \right)} + \pi$$
, where n - is a integer
Divide both parts of the equation by
$$\frac{1}{7}$$
$$x_{1} = 14 \pi n - 7 \operatorname{asin}{\left(\frac{1}{5} \right)}$$
$$x_{2} = 14 \pi n + 7 \operatorname{asin}{\left(\frac{1}{5} \right)} + 7 \pi$$
$$x_{1} = 14 \pi n - 7 \operatorname{asin}{\left(\frac{1}{5} \right)}$$
$$x_{2} = 14 \pi n + 7 \operatorname{asin}{\left(\frac{1}{5} \right)} + 7 \pi$$
This roots
$$x_{1} = 14 \pi n - 7 \operatorname{asin}{\left(\frac{1}{5} \right)}$$
$$x_{2} = 14 \pi n + 7 \operatorname{asin}{\left(\frac{1}{5} \right)} + 7 \pi$$
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
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$\left(14 \pi n - 7 \operatorname{asin}{\left(\frac{1}{5} \right)}\right) + - \frac{1}{10}$$
=
$$14 \pi n - 7 \operatorname{asin}{\left(\frac{1}{5} \right)} - \frac{1}{10}$$
substitute to the expression
$$\sin{\left(\frac{x}{7} \right)} > - \frac{1}{5}$$
$$\sin{\left(\frac{14 \pi n - 7 \operatorname{asin}{\left(\frac{1}{5} \right)} - \frac{1}{10}}{7} \right)} > - \frac{1}{5}$$
-sin(1/70 - 2*pi*n + asin(1/5)) > -1/5
Then
$$x < 14 \pi n - 7 \operatorname{asin}{\left(\frac{1}{5} \right)}$$
no execute
one of the solutions of our inequality is:
$$x > 14 \pi n - 7 \operatorname{asin}{\left(\frac{1}{5} \right)} \wedge x < 14 \pi n + 7 \operatorname{asin}{\left(\frac{1}{5} \right)} + 7 \pi$$
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