0.4x^2-0.4x=9.9

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Solution for 0.4x^2-0.4x=9.9 equation:


Simplifying
0.4x2 + -0.4x = 9.9

Reorder the terms:
-0.4x + 0.4x2 = 9.9

Solving
-0.4x + 0.4x2 = 9.9

Solving for variable 'x'.

Reorder the terms:
-9.9 + -0.4x + 0.4x2 = 9.9 + -9.9

Combine like terms: 9.9 + -9.9 = 0.0
-9.9 + -0.4x + 0.4x2 = 0.0

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

Divide each side by '0.4'.
-24.75 + -1x + x2 = 0

Move the constant term to the right:

Add '24.75' to each side of the equation.
-24.75 + -1x + 24.75 + x2 = 0 + 24.75

Reorder the terms:
-24.75 + 24.75 + -1x + x2 = 0 + 24.75

Combine like terms: -24.75 + 24.75 = 0.00
0.00 + -1x + x2 = 0 + 24.75
-1x + x2 = 0 + 24.75

Combine like terms: 0 + 24.75 = 24.75
-1x + x2 = 24.75

The x term is -1x.  Take half its coefficient (-0.5).
Square it (0.25) and add it to both sides.

Add '0.25' to each side of the equation.
-1x + 0.25 + x2 = 24.75 + 0.25

Reorder the terms:
0.25 + -1x + x2 = 24.75 + 0.25

Combine like terms: 24.75 + 0.25 = 25
0.25 + -1x + x2 = 25

Factor a perfect square on the left side:
(x + -0.5)(x + -0.5) = 25

Calculate the square root of the right side: 5

Break this problem into two subproblems by setting 
(x + -0.5) equal to 5 and -5.

Subproblem 1

x + -0.5 = 5 Simplifying x + -0.5 = 5 Reorder the terms: -0.5 + x = 5 Solving -0.5 + x = 5 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '0.5' to each side of the equation. -0.5 + 0.5 + x = 5 + 0.5 Combine like terms: -0.5 + 0.5 = 0.0 0.0 + x = 5 + 0.5 x = 5 + 0.5 Combine like terms: 5 + 0.5 = 5.5 x = 5.5 Simplifying x = 5.5

Subproblem 2

x + -0.5 = -5 Simplifying x + -0.5 = -5 Reorder the terms: -0.5 + x = -5 Solving -0.5 + x = -5 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '0.5' to each side of the equation. -0.5 + 0.5 + x = -5 + 0.5 Combine like terms: -0.5 + 0.5 = 0.0 0.0 + x = -5 + 0.5 x = -5 + 0.5 Combine like terms: -5 + 0.5 = -4.5 x = -4.5 Simplifying x = -4.5

Solution

The solution to the problem is based on the solutions from the subproblems. x = {5.5, -4.5}

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