2sin^2x−sinx−3<0 inequation

Given the inequality:
$$\left(2 \sin^{2}{\left(x \right)} - \sin{\left(x \right)}\right) - 3 < 0$$
To solve this inequality, we must first solve the corresponding equation:
$$\left(2 \sin^{2}{\left(x \right)} - \sin{\left(x \right)}\right) - 3 = 0$$
Solve:
Given the equation
$$\left(2 \sin^{2}{\left(x \right)} - \sin{\left(x \right)}\right) - 3 = 0$$
transform
$$2 \sin^{2}{\left(x \right)} - \sin{\left(x \right)} - 3 = 0$$
$$\left(2 \sin^{2}{\left(x \right)} - \sin{\left(x \right)}\right) - 3 = 0$$
Do replacement
$$w = \sin{\left(x \right)}$$
This equation is of the form

a*w^2 + b*w + c = 0

A quadratic equation can be solved
using the discriminant.
The roots of the quadratic equation:
$$w_{1} = \frac{\sqrt{D} - b}{2 a}$$
$$w_{2} = \frac{- \sqrt{D} - b}{2 a}$$
where D = b^2 - 4*a*c - it is the discriminant.
Because
$$a = 2$$
$$b = -1$$
$$c = -3$$
, then

D = b^2 - 4 * a * c = 

(-1)^2 - 4 * (2) * (-3) = 25

Because D > 0, then the equation has two roots.

w1 = (-b + sqrt(D)) / (2*a)

w2 = (-b - sqrt(D)) / (2*a)

or
$$w_{1} = \frac{3}{2}$$
$$w_{2} = -1$$
do backward replacement
$$\sin{\left(x \right)} = w$$
Given the equation
$$\sin{\left(x \right)} = w$$
- this is the simplest trigonometric equation
This equation is transformed to
$$x = 2 \pi n + \operatorname{asin}{\left(w \right)}$$
$$x = 2 \pi n - \operatorname{asin}{\left(w \right)} + \pi$$
Or
$$x = 2 \pi n + \operatorname{asin}{\left(w \right)}$$
$$x = 2 \pi n - \operatorname{asin}{\left(w \right)} + \pi$$
, where n - is a integer
substitute w:
$$x_{1} = 2 \pi n + \operatorname{asin}{\left(w_{1} \right)}$$
$$x_{1} = 2 \pi n + \operatorname{asin}{\left(\frac{3}{2} \right)}$$
$$x_{1} = 2 \pi n + \operatorname{asin}{\left(\frac{3}{2} \right)}$$
$$x_{2} = 2 \pi n + \operatorname{asin}{\left(w_{2} \right)}$$
$$x_{2} = 2 \pi n + \operatorname{asin}{\left(-1 \right)}$$
$$x_{2} = 2 \pi n - \frac{\pi}{2}$$
$$x_{3} = 2 \pi n - \operatorname{asin}{\left(w_{1} \right)} + \pi$$
$$x_{3} = 2 \pi n + \pi - \operatorname{asin}{\left(\frac{3}{2} \right)}$$
$$x_{3} = 2 \pi n + \pi - \operatorname{asin}{\left(\frac{3}{2} \right)}$$
$$x_{4} = 2 \pi n - \operatorname{asin}{\left(w_{2} \right)} + \pi$$
$$x_{4} = 2 \pi n - \operatorname{asin}{\left(-1 \right)} + \pi$$
$$x_{4} = 2 \pi n + \frac{3 \pi}{2}$$
$$x_{1} = - \frac{\pi}{2}$$
$$x_{2} = \frac{3 \pi}{2}$$
$$x_{3} = \pi - \operatorname{asin}{\left(\frac{3}{2} \right)}$$
$$x_{4} = \operatorname{asin}{\left(\frac{3}{2} \right)}$$
Exclude the complex solutions:
$$x_{1} = - \frac{\pi}{2}$$
$$x_{2} = \frac{3 \pi}{2}$$
This roots
$$x_{1} = - \frac{\pi}{2}$$
$$x_{2} = \frac{3 \pi}{2}$$
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
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$- \frac{\pi}{2} - \frac{1}{10}$$
=
$$- \frac{\pi}{2} - \frac{1}{10}$$
substitute to the expression
$$\left(2 \sin^{2}{\left(x \right)} - \sin{\left(x \right)}\right) - 3 < 0$$
$$-3 + \left(- \sin{\left(- \frac{\pi}{2} - \frac{1}{10} \right)} + 2 \sin^{2}{\left(- \frac{\pi}{2} - \frac{1}{10} \right)}\right) < 0$$

          2                      
-3 + 2*cos (1/10) + cos(1/10) < 0
    

one of the solutions of our inequality is:
$$x < - \frac{\pi}{2}$$

 _____           _____          
      \         /
-------ο-------ο-------
       x1      x2

Other solutions will get with the changeover to the next point
etc.
The answer:
$$x < - \frac{\pi}{2}$$
$$x > \frac{3 \pi}{2}$$