0.4x(5+2x)-0.5=0.9x+2.1

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Solution for 0.4x(5+2x)-0.5=0.9x+2.1 equation:


Simplifying
0.4x(5 + 2x) + -0.5 = 0.9x + 2.1
(5 * 0.4x + 2x * 0.4x) + -0.5 = 0.9x + 2.1
(2x + 0.8x2) + -0.5 = 0.9x + 2.1

Reorder the terms:
-0.5 + 2x + 0.8x2 = 0.9x + 2.1

Reorder the terms:
-0.5 + 2x + 0.8x2 = 2.1 + 0.9x

Solving
-0.5 + 2x + 0.8x2 = 2.1 + 0.9x

Solving for variable 'x'.

Reorder the terms:
-0.5 + -2.1 + 2x + -0.9x + 0.8x2 = 2.1 + 0.9x + -2.1 + -0.9x

Combine like terms: -0.5 + -2.1 = -2.6
-2.6 + 2x + -0.9x + 0.8x2 = 2.1 + 0.9x + -2.1 + -0.9x

Combine like terms: 2x + -0.9x = 1.1x
-2.6 + 1.1x + 0.8x2 = 2.1 + 0.9x + -2.1 + -0.9x

Reorder the terms:
-2.6 + 1.1x + 0.8x2 = 2.1 + -2.1 + 0.9x + -0.9x

Combine like terms: 2.1 + -2.1 = 0.0
-2.6 + 1.1x + 0.8x2 = 0.0 + 0.9x + -0.9x
-2.6 + 1.1x + 0.8x2 = 0.9x + -0.9x

Combine like terms: 0.9x + -0.9x = 0.0
-2.6 + 1.1x + 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'.
-3.25 + 1.375x + x2 = 0

Move the constant term to the right:

Add '3.25' to each side of the equation.
-3.25 + 1.375x + 3.25 + x2 = 0 + 3.25

Reorder the terms:
-3.25 + 3.25 + 1.375x + x2 = 0 + 3.25

Combine like terms: -3.25 + 3.25 = 0.00
0.00 + 1.375x + x2 = 0 + 3.25
1.375x + x2 = 0 + 3.25

Combine like terms: 0 + 3.25 = 3.25
1.375x + x2 = 3.25

The x term is 1.375x.  Take half its coefficient (0.6875).
Square it (0.47265625) and add it to both sides.

Add '0.47265625' to each side of the equation.
1.375x + 0.47265625 + x2 = 3.25 + 0.47265625

Reorder the terms:
0.47265625 + 1.375x + x2 = 3.25 + 0.47265625

Combine like terms: 3.25 + 0.47265625 = 3.72265625
0.47265625 + 1.375x + x2 = 3.72265625

Factor a perfect square on the left side:
(x + 0.6875)(x + 0.6875) = 3.72265625

Calculate the square root of the right side: 1.92941863

Break this problem into two subproblems by setting 
(x + 0.6875) equal to 1.92941863 and -1.92941863.

Subproblem 1

x + 0.6875 = 1.92941863 Simplifying x + 0.6875 = 1.92941863 Reorder the terms: 0.6875 + x = 1.92941863 Solving 0.6875 + x = 1.92941863 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-0.6875' to each side of the equation. 0.6875 + -0.6875 + x = 1.92941863 + -0.6875 Combine like terms: 0.6875 + -0.6875 = 0.0000 0.0000 + x = 1.92941863 + -0.6875 x = 1.92941863 + -0.6875 Combine like terms: 1.92941863 + -0.6875 = 1.24191863 x = 1.24191863 Simplifying x = 1.24191863

Subproblem 2

x + 0.6875 = -1.92941863 Simplifying x + 0.6875 = -1.92941863 Reorder the terms: 0.6875 + x = -1.92941863 Solving 0.6875 + x = -1.92941863 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-0.6875' to each side of the equation. 0.6875 + -0.6875 + x = -1.92941863 + -0.6875 Combine like terms: 0.6875 + -0.6875 = 0.0000 0.0000 + x = -1.92941863 + -0.6875 x = -1.92941863 + -0.6875 Combine like terms: -1.92941863 + -0.6875 = -2.61691863 x = -2.61691863 Simplifying x = -2.61691863

Solution

The solution to the problem is based on the solutions from the subproblems. x = {1.24191863, -2.61691863}

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