Given the inequality:
$$\cos{\left(x \right)} > \sin{\left(2 x \right)}$$
To solve this inequality, we must first solve the corresponding equation:
$$\cos{\left(x \right)} = \sin{\left(2 x \right)}$$
Solve:
$$x_{1} = - \frac{\pi}{2}$$
$$x_{2} = \frac{\pi}{6}$$
$$x_{3} = \frac{\pi}{2}$$
$$x_{4} = \frac{5 \pi}{6}$$
$$x_{1} = - \frac{\pi}{2}$$
$$x_{2} = \frac{\pi}{6}$$
$$x_{3} = \frac{\pi}{2}$$
$$x_{4} = \frac{5 \pi}{6}$$
This roots
$$x_{1} = - \frac{\pi}{2}$$
$$x_{2} = \frac{\pi}{6}$$
$$x_{3} = \frac{\pi}{2}$$
$$x_{4} = \frac{5 \pi}{6}$$
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
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$- \frac{\pi}{2} - \frac{1}{10}$$
=
$$- \frac{\pi}{2} - \frac{1}{10}$$
substitute to the expression
$$\cos{\left(x \right)} > \sin{\left(2 x \right)}$$
$$\cos{\left(- \frac{\pi}{2} - \frac{1}{10} \right)} > \sin{\left(2 \left(- \frac{\pi}{2} - \frac{1}{10}\right) \right)}$$
-sin(1/10) > sin(1/5)
Then
$$x < - \frac{\pi}{2}$$
no execute
one of the solutions of our inequality is:
$$x > - \frac{\pi}{2} \wedge x < \frac{\pi}{6}$$
_____ _____
/ \ / \
-------ο-------ο-------ο-------ο-------
x_1 x_2 x_3 x_4
Other solutions will get with the changeover to the next point
etc.
The answer:
$$x > - \frac{\pi}{2} \wedge x < \frac{\pi}{6}$$
$$x > \frac{\pi}{2} \wedge x < \frac{5 \pi}{6}$$