Mister Exam

cosx>sin2x inequation

A inequation with variable

The solution

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cos(x) > sin(2*x)
$$\cos{\left(x \right)} > \sin{\left(2 x \right)}$$
cos(x) > sin(2*x)
Detail solution
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:
$$x > - \frac{\pi}{2} \wedge x < \frac{\pi}{6}$$
$$x > \frac{\pi}{2} \wedge x < \frac{5 \pi}{6}$$
Solving inequality on a graph
Rapid solution [src]
  /   /            pi\     /pi          5*pi\     /3*pi              \\
Or|And|0 <= x, x < --|, And|-- < x, x < ----|, And|---- < x, x < 2*pi||
  \   \            6 /     \2            6  /     \ 2                //
$$\left(0 \leq x \wedge x < \frac{\pi}{6}\right) \vee \left(\frac{\pi}{2} < x \wedge x < \frac{5 \pi}{6}\right) \vee \left(\frac{3 \pi}{2} < x \wedge x < 2 \pi\right)$$
((0 <= x)∧(x < pi/6))∨((pi/2 < x)∧(x < 5*pi/6))∨((3*pi/2 < x)∧(x < 2*pi))
Rapid solution 2 [src]
    pi     pi  5*pi     3*pi       
[0, --) U (--, ----) U (----, 2*pi)
    6      2    6        2         
$$x\ in\ \left[0, \frac{\pi}{6}\right) \cup \left(\frac{\pi}{2}, \frac{5 \pi}{6}\right) \cup \left(\frac{3 \pi}{2}, 2 \pi\right)$$
x in Union(Interval.Ropen(0, pi/6), Interval.open(pi/2, 5*pi/6), Interval.open(3*pi/2, 2*pi))
The graph
cosx>sin2x inequation