Given the inequality:
$$3 \sin^{2}{\left(2 x \right)} + 7 \cos{\left(2 x \right)} - 3 \geq 0$$
To solve this inequality, we must first solve the corresponding equation:
$$3 \sin^{2}{\left(2 x \right)} + 7 \cos{\left(2 x \right)} - 3 = 0$$
Solve:
$$x_{1} = - \frac{\pi}{4}$$
$$x_{2} = \frac{\pi}{4}$$
$$x_{3} = - i \operatorname{atanh}{\left(\frac{\sqrt{10}}{5} \right)}$$
$$x_{4} = i \operatorname{atanh}{\left(\frac{\sqrt{10}}{5} \right)}$$
Exclude the complex solutions:
$$x_{1} = - \frac{\pi}{4}$$
$$x_{2} = \frac{\pi}{4}$$
This roots
$$x_{1} = - \frac{\pi}{4}$$
$$x_{2} = \frac{\pi}{4}$$
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
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$- \frac{\pi}{4} - \frac{1}{10}$$
=
$$- \frac{\pi}{4} - \frac{1}{10}$$
substitute to the expression
$$3 \sin^{2}{\left(2 x \right)} + 7 \cos{\left(2 x \right)} - 3 \geq 0$$
$$\left(-1\right) 3 + 7 \cos{\left(2 \left(- \frac{\pi}{4} - \frac{1}{10}\right) \right)} + 3 \sin^{2}{\left(2 \left(- \frac{\pi}{4} - \frac{1}{10}\right) \right)} \geq 0$$
2
-3 - 7*sin(1/5) + 3*cos (1/5) >= 0
but
2
-3 - 7*sin(1/5) + 3*cos (1/5) < 0
Then
$$x \leq - \frac{\pi}{4}$$
no execute
one of the solutions of our inequality is:
$$x \geq - \frac{\pi}{4} \wedge x \leq \frac{\pi}{4}$$
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