5=0.8x(x-9)

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Solution for 5=0.8x(x-9) equation:


Simplifying
5 = 0.8x(x + -9)

Reorder the terms:
5 = 0.8x(-9 + x)
5 = (-9 * 0.8x + x * 0.8x)
5 = (-7.2x + 0.8x2)

Solving
5 = -7.2x + 0.8x2

Solving for variable 'x'.

Reorder the terms:
5 + 7.2x + -0.8x2 = -7.2x + 7.2x + 0.8x2 + -0.8x2

Combine like terms: -7.2x + 7.2x = 0.0
5 + 7.2x + -0.8x2 = 0.0 + 0.8x2 + -0.8x2
5 + 7.2x + -0.8x2 = 0.8x2 + -0.8x2

Combine like terms: 0.8x2 + -0.8x2 = 0.0
5 + 7.2x + -0.8x2 = 0.0

Begin completing the square.  Divide all terms by
-0.8 the coefficient of the squared term: 

Divide each side by '-0.8'.
-6.25 + -9x + x2 = 0

Move the constant term to the right:

Add '6.25' to each side of the equation.
-6.25 + -9x + 6.25 + x2 = 0 + 6.25

Reorder the terms:
-6.25 + 6.25 + -9x + x2 = 0 + 6.25

Combine like terms: -6.25 + 6.25 = 0.00
0.00 + -9x + x2 = 0 + 6.25
-9x + x2 = 0 + 6.25

Combine like terms: 0 + 6.25 = 6.25
-9x + x2 = 6.25

The x term is -9x.  Take half its coefficient (-4.5).
Square it (20.25) and add it to both sides.

Add '20.25' to each side of the equation.
-9x + 20.25 + x2 = 6.25 + 20.25

Reorder the terms:
20.25 + -9x + x2 = 6.25 + 20.25

Combine like terms: 6.25 + 20.25 = 26.5
20.25 + -9x + x2 = 26.5

Factor a perfect square on the left side:
(x + -4.5)(x + -4.5) = 26.5

Calculate the square root of the right side: 5.14781507

Break this problem into two subproblems by setting 
(x + -4.5) equal to 5.14781507 and -5.14781507.

Subproblem 1

x + -4.5 = 5.14781507 Simplifying x + -4.5 = 5.14781507 Reorder the terms: -4.5 + x = 5.14781507 Solving -4.5 + x = 5.14781507 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '4.5' to each side of the equation. -4.5 + 4.5 + x = 5.14781507 + 4.5 Combine like terms: -4.5 + 4.5 = 0.0 0.0 + x = 5.14781507 + 4.5 x = 5.14781507 + 4.5 Combine like terms: 5.14781507 + 4.5 = 9.64781507 x = 9.64781507 Simplifying x = 9.64781507

Subproblem 2

x + -4.5 = -5.14781507 Simplifying x + -4.5 = -5.14781507 Reorder the terms: -4.5 + x = -5.14781507 Solving -4.5 + x = -5.14781507 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '4.5' to each side of the equation. -4.5 + 4.5 + x = -5.14781507 + 4.5 Combine like terms: -4.5 + 4.5 = 0.0 0.0 + x = -5.14781507 + 4.5 x = -5.14781507 + 4.5 Combine like terms: -5.14781507 + 4.5 = -0.64781507 x = -0.64781507 Simplifying x = -0.64781507

Solution

The solution to the problem is based on the solutions from the subproblems. x = {9.64781507, -0.64781507}

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