x^2+21x=-21

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Solution for x^2+21x=-21 equation:


Simplifying
x2 + 21x = -21

Reorder the terms:
21x + x2 = -21

Solving
21x + x2 = -21

Solving for variable 'x'.

Reorder the terms:
21 + 21x + x2 = -21 + 21

Combine like terms: -21 + 21 = 0
21 + 21x + x2 = 0

Begin completing the square.

Move the constant term to the right:

Add '-21' to each side of the equation.
21 + 21x + -21 + x2 = 0 + -21

Reorder the terms:
21 + -21 + 21x + x2 = 0 + -21

Combine like terms: 21 + -21 = 0
0 + 21x + x2 = 0 + -21
21x + x2 = 0 + -21

Combine like terms: 0 + -21 = -21
21x + x2 = -21

The x term is 21x.  Take half its coefficient (10.5).
Square it (110.25) and add it to both sides.

Add '110.25' to each side of the equation.
21x + 110.25 + x2 = -21 + 110.25

Reorder the terms:
110.25 + 21x + x2 = -21 + 110.25

Combine like terms: -21 + 110.25 = 89.25
110.25 + 21x + x2 = 89.25

Factor a perfect square on the left side:
(x + 10.5)(x + 10.5) = 89.25

Calculate the square root of the right side: 9.447221814

Break this problem into two subproblems by setting 
(x + 10.5) equal to 9.447221814 and -9.447221814.

Subproblem 1

x + 10.5 = 9.447221814 Simplifying x + 10.5 = 9.447221814 Reorder the terms: 10.5 + x = 9.447221814 Solving 10.5 + x = 9.447221814 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-10.5' to each side of the equation. 10.5 + -10.5 + x = 9.447221814 + -10.5 Combine like terms: 10.5 + -10.5 = 0.0 0.0 + x = 9.447221814 + -10.5 x = 9.447221814 + -10.5 Combine like terms: 9.447221814 + -10.5 = -1.052778186 x = -1.052778186 Simplifying x = -1.052778186

Subproblem 2

x + 10.5 = -9.447221814 Simplifying x + 10.5 = -9.447221814 Reorder the terms: 10.5 + x = -9.447221814 Solving 10.5 + x = -9.447221814 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-10.5' to each side of the equation. 10.5 + -10.5 + x = -9.447221814 + -10.5 Combine like terms: 10.5 + -10.5 = 0.0 0.0 + x = -9.447221814 + -10.5 x = -9.447221814 + -10.5 Combine like terms: -9.447221814 + -10.5 = -19.947221814 x = -19.947221814 Simplifying x = -19.947221814

Solution

The solution to the problem is based on the solutions from the subproblems. x = {-1.052778186, -19.947221814}

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