.4x(8-.2x)=-4

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Solution for .4x(8-.2x)=-4 equation:


Simplifying
0.4x(8 + -0.2x) = -4
(8 * 0.4x + -0.2x * 0.4x) = -4
(3.2x + -0.08x2) = -4

Solving
3.2x + -0.08x2 = -4

Solving for variable 'x'.

Reorder the terms:
4 + 3.2x + -0.08x2 = -4 + 4

Combine like terms: -4 + 4 = 0
4 + 3.2x + -0.08x2 = 0

Begin completing the square.  Divide all terms by
-0.08 the coefficient of the squared term: 

Divide each side by '-0.08'.
-50 + -40x + x2 = 0

Move the constant term to the right:

Add '50' to each side of the equation.
-50 + -40x + 50 + x2 = 0 + 50

Reorder the terms:
-50 + 50 + -40x + x2 = 0 + 50

Combine like terms: -50 + 50 = 0
0 + -40x + x2 = 0 + 50
-40x + x2 = 0 + 50

Combine like terms: 0 + 50 = 50
-40x + x2 = 50

The x term is -40x.  Take half its coefficient (-20).
Square it (400) and add it to both sides.

Add '400' to each side of the equation.
-40x + 400 + x2 = 50 + 400

Reorder the terms:
400 + -40x + x2 = 50 + 400

Combine like terms: 50 + 400 = 450
400 + -40x + x2 = 450

Factor a perfect square on the left side:
(x + -20)(x + -20) = 450

Calculate the square root of the right side: 21.213203436

Break this problem into two subproblems by setting 
(x + -20) equal to 21.213203436 and -21.213203436.

Subproblem 1

x + -20 = 21.213203436 Simplifying x + -20 = 21.213203436 Reorder the terms: -20 + x = 21.213203436 Solving -20 + x = 21.213203436 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '20' to each side of the equation. -20 + 20 + x = 21.213203436 + 20 Combine like terms: -20 + 20 = 0 0 + x = 21.213203436 + 20 x = 21.213203436 + 20 Combine like terms: 21.213203436 + 20 = 41.213203436 x = 41.213203436 Simplifying x = 41.213203436

Subproblem 2

x + -20 = -21.213203436 Simplifying x + -20 = -21.213203436 Reorder the terms: -20 + x = -21.213203436 Solving -20 + x = -21.213203436 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '20' to each side of the equation. -20 + 20 + x = -21.213203436 + 20 Combine like terms: -20 + 20 = 0 0 + x = -21.213203436 + 20 x = -21.213203436 + 20 Combine like terms: -21.213203436 + 20 = -1.213203436 x = -1.213203436 Simplifying x = -1.213203436

Solution

The solution to the problem is based on the solutions from the subproblems. x = {41.213203436, -1.213203436}

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