2x^2+2x=21

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


Simplifying
2x2 + 2x = 21

Reorder the terms:
2x + 2x2 = 21

Solving
2x + 2x2 = 21

Solving for variable 'x'.

Reorder the terms:
-21 + 2x + 2x2 = 21 + -21

Combine like terms: 21 + -21 = 0
-21 + 2x + 2x2 = 0

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

Divide each side by '2'.
-10.5 + x + x2 = 0

Move the constant term to the right:

Add '10.5' to each side of the equation.
-10.5 + x + 10.5 + x2 = 0 + 10.5

Reorder the terms:
-10.5 + 10.5 + x + x2 = 0 + 10.5

Combine like terms: -10.5 + 10.5 = 0.0
0.0 + x + x2 = 0 + 10.5
x + x2 = 0 + 10.5

Combine like terms: 0 + 10.5 = 10.5
x + x2 = 10.5

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.
 + 0.25 + x2 = 10.5 + 0.25

Combine like terms:  + 0.25 = 1.25
1.25 + x2 = 10.5 + 0.25

Combine like terms: 10.5 + 0.25 = 10.75
1.25 + x2 = 10.75

Factor a perfect square on the left side:
(x + 0.5)(x + 0.5) = 10.75

Calculate the square root of the right side: 3.278719262

Break this problem into two subproblems by setting 
(x + 0.5) equal to 3.278719262 and -3.278719262.

Subproblem 1

x + 0.5 = 3.278719262 Simplifying x + 0.5 = 3.278719262 Reorder the terms: 0.5 + x = 3.278719262 Solving 0.5 + x = 3.278719262 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 = 3.278719262 + -0.5 Combine like terms: 0.5 + -0.5 = 0.0 0.0 + x = 3.278719262 + -0.5 x = 3.278719262 + -0.5 Combine like terms: 3.278719262 + -0.5 = 2.778719262 x = 2.778719262 Simplifying x = 2.778719262

Subproblem 2

x + 0.5 = -3.278719262 Simplifying x + 0.5 = -3.278719262 Reorder the terms: 0.5 + x = -3.278719262 Solving 0.5 + x = -3.278719262 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 = -3.278719262 + -0.5 Combine like terms: 0.5 + -0.5 = 0.0 0.0 + x = -3.278719262 + -0.5 x = -3.278719262 + -0.5 Combine like terms: -3.278719262 + -0.5 = -3.778719262 x = -3.778719262 Simplifying x = -3.778719262

Solution

The solution to the problem is based on the solutions from the subproblems. x = {2.778719262, -3.778719262}

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