(4x+5)(x)=400

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Solution for (4x+5)(x)=400 equation:


Simplifying
(4x + 5)(x) = 400

Reorder the terms:
(5 + 4x)(x) = 400

Reorder the terms for easier multiplication:
x(5 + 4x) = 400
(5 * x + 4x * x) = 400
(5x + 4x2) = 400

Solving
5x + 4x2 = 400

Solving for variable 'x'.

Reorder the terms:
-400 + 5x + 4x2 = 400 + -400

Combine like terms: 400 + -400 = 0
-400 + 5x + 4x2 = 0

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

Divide each side by '4'.
-100 + 1.25x + x2 = 0

Move the constant term to the right:

Add '100' to each side of the equation.
-100 + 1.25x + 100 + x2 = 0 + 100

Reorder the terms:
-100 + 100 + 1.25x + x2 = 0 + 100

Combine like terms: -100 + 100 = 0
0 + 1.25x + x2 = 0 + 100
1.25x + x2 = 0 + 100

Combine like terms: 0 + 100 = 100
1.25x + x2 = 100

The x term is 1.25x.  Take half its coefficient (0.625).
Square it (0.390625) and add it to both sides.

Add '0.390625' to each side of the equation.
1.25x + 0.390625 + x2 = 100 + 0.390625

Reorder the terms:
0.390625 + 1.25x + x2 = 100 + 0.390625

Combine like terms: 100 + 0.390625 = 100.390625
0.390625 + 1.25x + x2 = 100.390625

Factor a perfect square on the left side:
(x + 0.625)(x + 0.625) = 100.390625

Calculate the square root of the right side: 10.019512214

Break this problem into two subproblems by setting 
(x + 0.625) equal to 10.019512214 and -10.019512214.

Subproblem 1

x + 0.625 = 10.019512214 Simplifying x + 0.625 = 10.019512214 Reorder the terms: 0.625 + x = 10.019512214 Solving 0.625 + x = 10.019512214 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-0.625' to each side of the equation. 0.625 + -0.625 + x = 10.019512214 + -0.625 Combine like terms: 0.625 + -0.625 = 0.000 0.000 + x = 10.019512214 + -0.625 x = 10.019512214 + -0.625 Combine like terms: 10.019512214 + -0.625 = 9.394512214 x = 9.394512214 Simplifying x = 9.394512214

Subproblem 2

x + 0.625 = -10.019512214 Simplifying x + 0.625 = -10.019512214 Reorder the terms: 0.625 + x = -10.019512214 Solving 0.625 + x = -10.019512214 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-0.625' to each side of the equation. 0.625 + -0.625 + x = -10.019512214 + -0.625 Combine like terms: 0.625 + -0.625 = 0.000 0.000 + x = -10.019512214 + -0.625 x = -10.019512214 + -0.625 Combine like terms: -10.019512214 + -0.625 = -10.644512214 x = -10.644512214 Simplifying x = -10.644512214

Solution

The solution to the problem is based on the solutions from the subproblems. x = {9.394512214, -10.644512214}

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