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-3*0,3904*x^2-0,888*2*x+6,6932=0 equation

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Numerical solution:

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The solution

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          2   111*2               
- 1.1712*x  - -----*x + 6.6932 = 0
               125                
$$\left(- 1.1712 x^{2} - \frac{2 \cdot 111}{125} x\right) + 6.6932 = 0$$
Detail solution
This equation is of the form
a*x^2 + b*x + c = 0

A quadratic equation can be solved
using the discriminant.
The roots of the quadratic equation:
$$x_{1} = \frac{\sqrt{D} - b}{2 a}$$
$$x_{2} = \frac{- \sqrt{D} - b}{2 a}$$
where D = b^2 - 4*a*c - it is the discriminant.
Because
$$a = -1.1712$$
$$b = - \frac{222}{125}$$
$$c = 6.6932$$
, then
D = b^2 - 4 * a * c = 

(-222/125)^2 - 4 * (-1.1712) * (6.6932) = 34.5104793600000

Because D > 0, then the equation has two roots.
x1 = (-b + sqrt(D)) / (2*a)

x2 = (-b - sqrt(D)) / (2*a)

or
$$x_{1} = -3.26612109668228$$
$$x_{2} = 1.74972765405933$$
Vieta's Theorem
rewrite the equation
$$\left(- 1.1712 x^{2} - \frac{2 \cdot 111}{125} x\right) + 6.6932 = 0$$
of
$$a x^{2} + b x + c = 0$$
as reduced quadratic equation
$$x^{2} + \frac{b x}{a} + \frac{c}{a} = 0$$
$$1 x^{2} + 1.51639344262295 x - 5.71482240437158 = 0$$
$$p x + q + x^{2} = 0$$
where
$$p = \frac{b}{a}$$
$$p = 1.51639344262295$$
$$q = \frac{c}{a}$$
$$q = -5.71482240437158$$
Vieta Formulas
$$x_{1} + x_{2} = - p$$
$$x_{1} x_{2} = q$$
$$x_{1} + x_{2} = -1.51639344262295$$
$$x_{1} x_{2} = -5.71482240437158$$
The graph
Rapid solution [src]
x1 = -3.26612109668228
$$x_{1} = -3.26612109668228$$
x2 = 1.74972765405933
$$x_{2} = 1.74972765405933$$
x2 = 1.74972765405933
Sum and product of roots [src]
sum
-3.26612109668228 + 1.74972765405933
$$-3.26612109668228 + 1.74972765405933$$
=
-1.51639344262295
$$-1.51639344262295$$
product
-3.26612109668228*1.74972765405933
$$- 1.74972765405933 \cdot 3.26612109668228$$
=
-5.71482240437158
$$-5.71482240437158$$
-5.71482240437158
Numerical answer [src]
x1 = 1.74972765405933
x2 = -3.26612109668228
x2 = -3.26612109668228