x^2+x-8=391

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Solution for x^2+x-8=391 equation:


Simplifying
x2 + x + -8 = 391

Reorder the terms:
-8 + x + x2 = 391

Solving
-8 + x + x2 = 391

Solving for variable 'x'.

Reorder the terms:
-8 + -391 + x + x2 = 391 + -391

Combine like terms: -8 + -391 = -399
-399 + x + x2 = 391 + -391

Combine like terms: 391 + -391 = 0
-399 + x + x2 = 0

Begin completing the square.

Move the constant term to the right:

Add '399' to each side of the equation.
-399 + x + 399 + x2 = 0 + 399

Reorder the terms:
-399 + 399 + x + x2 = 0 + 399

Combine like terms: -399 + 399 = 0
0 + x + x2 = 0 + 399
x + x2 = 0 + 399

Combine like terms: 0 + 399 = 399
x + x2 = 399

The x term is x.  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.
x + 0.25 + x2 = 399 + 0.25

Reorder the terms:
0.25 + x + x2 = 399 + 0.25

Combine like terms: 399 + 0.25 = 399.25
0.25 + x + x2 = 399.25

Factor a perfect square on the left side:
(x + 0.5)(x + 0.5) = 399.25

Calculate the square root of the right side: 19.981241203

Break this problem into two subproblems by setting 
(x + 0.5) equal to 19.981241203 and -19.981241203.

Subproblem 1

x + 0.5 = 19.981241203 Simplifying x + 0.5 = 19.981241203 Reorder the terms: 0.5 + x = 19.981241203 Solving 0.5 + x = 19.981241203 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 = 19.981241203 + -0.5 Combine like terms: 0.5 + -0.5 = 0.0 0.0 + x = 19.981241203 + -0.5 x = 19.981241203 + -0.5 Combine like terms: 19.981241203 + -0.5 = 19.481241203 x = 19.481241203 Simplifying x = 19.481241203

Subproblem 2

x + 0.5 = -19.981241203 Simplifying x + 0.5 = -19.981241203 Reorder the terms: 0.5 + x = -19.981241203 Solving 0.5 + x = -19.981241203 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 = -19.981241203 + -0.5 Combine like terms: 0.5 + -0.5 = 0.0 0.0 + x = -19.981241203 + -0.5 x = -19.981241203 + -0.5 Combine like terms: -19.981241203 + -0.5 = -20.481241203 x = -20.481241203 Simplifying x = -20.481241203

Solution

The solution to the problem is based on the solutions from the subproblems. x = {19.481241203, -20.481241203}

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