0=.21x^2-.4x-1.6

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Solution for 0=.21x^2-.4x-1.6 equation:


Simplifying
0 = 0.21x2 + -0.4x + -1.6

Reorder the terms:
0 = -1.6 + -0.4x + 0.21x2

Solving
0 = -1.6 + -0.4x + 0.21x2

Solving for variable 'x'.

Combine like terms: 0 + 1.6 = 1.6
1.6 + 0.4x + -0.21x2 = -1.6 + -0.4x + 0.21x2 + 1.6 + 0.4x + -0.21x2

Reorder the terms:
1.6 + 0.4x + -0.21x2 = -1.6 + 1.6 + -0.4x + 0.4x + 0.21x2 + -0.21x2

Combine like terms: -1.6 + 1.6 = 0.0
1.6 + 0.4x + -0.21x2 = 0.0 + -0.4x + 0.4x + 0.21x2 + -0.21x2
1.6 + 0.4x + -0.21x2 = -0.4x + 0.4x + 0.21x2 + -0.21x2

Combine like terms: -0.4x + 0.4x = 0.0
1.6 + 0.4x + -0.21x2 = 0.0 + 0.21x2 + -0.21x2
1.6 + 0.4x + -0.21x2 = 0.21x2 + -0.21x2

Combine like terms: 0.21x2 + -0.21x2 = 0.00
1.6 + 0.4x + -0.21x2 = 0.00

Begin completing the square.  Divide all terms by
-0.21 the coefficient of the squared term: 

Divide each side by '-0.21'.
-7.619047619 + -1.904761905x + x2 = 0

Move the constant term to the right:

Add '7.619047619' to each side of the equation.
-7.619047619 + -1.904761905x + 7.619047619 + x2 = 0 + 7.619047619

Reorder the terms:
-7.619047619 + 7.619047619 + -1.904761905x + x2 = 0 + 7.619047619

Combine like terms: -7.619047619 + 7.619047619 = 0.000000000
0.000000000 + -1.904761905x + x2 = 0 + 7.619047619
-1.904761905x + x2 = 0 + 7.619047619

Combine like terms: 0 + 7.619047619 = 7.619047619
-1.904761905x + x2 = 7.619047619

The x term is -1.904761905x.  Take half its coefficient (-0.9523809525).
Square it (0.9070294787) and add it to both sides.

Add '0.9070294787' to each side of the equation.
-1.904761905x + 0.9070294787 + x2 = 7.619047619 + 0.9070294787

Reorder the terms:
0.9070294787 + -1.904761905x + x2 = 7.619047619 + 0.9070294787

Combine like terms: 7.619047619 + 0.9070294787 = 8.5260770977
0.9070294787 + -1.904761905x + x2 = 8.5260770977

Factor a perfect square on the left side:
(x + -0.9523809525)(x + -0.9523809525) = 8.5260770977

Calculate the square root of the right side: 2.919944708

Break this problem into two subproblems by setting 
(x + -0.9523809525) equal to 2.919944708 and -2.919944708.

Subproblem 1

x + -0.9523809525 = 2.919944708 Simplifying x + -0.9523809525 = 2.919944708 Reorder the terms: -0.9523809525 + x = 2.919944708 Solving -0.9523809525 + x = 2.919944708 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '0.9523809525' to each side of the equation. -0.9523809525 + 0.9523809525 + x = 2.919944708 + 0.9523809525 Combine like terms: -0.9523809525 + 0.9523809525 = 0.0000000000 0.0000000000 + x = 2.919944708 + 0.9523809525 x = 2.919944708 + 0.9523809525 Combine like terms: 2.919944708 + 0.9523809525 = 3.8723256605 x = 3.8723256605 Simplifying x = 3.8723256605

Subproblem 2

x + -0.9523809525 = -2.919944708 Simplifying x + -0.9523809525 = -2.919944708 Reorder the terms: -0.9523809525 + x = -2.919944708 Solving -0.9523809525 + x = -2.919944708 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '0.9523809525' to each side of the equation. -0.9523809525 + 0.9523809525 + x = -2.919944708 + 0.9523809525 Combine like terms: -0.9523809525 + 0.9523809525 = 0.0000000000 0.0000000000 + x = -2.919944708 + 0.9523809525 x = -2.919944708 + 0.9523809525 Combine like terms: -2.919944708 + 0.9523809525 = -1.9675637555 x = -1.9675637555 Simplifying x = -1.9675637555

Solution

The solution to the problem is based on the solutions from the subproblems. x = {3.8723256605, -1.9675637555}

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