x^2+5x=21

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Solution for x^2+5x=21 equation:


Simplifying
x2 + 5x = 21

Reorder the terms:
5x + x2 = 21

Solving
5x + x2 = 21

Solving for variable 'x'.

Reorder the terms:
-21 + 5x + x2 = 21 + -21

Combine like terms: 21 + -21 = 0
-21 + 5x + x2 = 0

Begin completing the square.

Move the constant term to the right:

Add '21' to each side of the equation.
-21 + 5x + 21 + x2 = 0 + 21

Reorder the terms:
-21 + 21 + 5x + x2 = 0 + 21

Combine like terms: -21 + 21 = 0
0 + 5x + x2 = 0 + 21
5x + x2 = 0 + 21

Combine like terms: 0 + 21 = 21
5x + x2 = 21

The x term is 5x.  Take half its coefficient (2.5).
Square it (6.25) and add it to both sides.

Add '6.25' to each side of the equation.
5x + 6.25 + x2 = 21 + 6.25

Reorder the terms:
6.25 + 5x + x2 = 21 + 6.25

Combine like terms: 21 + 6.25 = 27.25
6.25 + 5x + x2 = 27.25

Factor a perfect square on the left side:
(x + 2.5)(x + 2.5) = 27.25

Calculate the square root of the right side: 5.220153254

Break this problem into two subproblems by setting 
(x + 2.5) equal to 5.220153254 and -5.220153254.

Subproblem 1

x + 2.5 = 5.220153254 Simplifying x + 2.5 = 5.220153254 Reorder the terms: 2.5 + x = 5.220153254 Solving 2.5 + x = 5.220153254 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-2.5' to each side of the equation. 2.5 + -2.5 + x = 5.220153254 + -2.5 Combine like terms: 2.5 + -2.5 = 0.0 0.0 + x = 5.220153254 + -2.5 x = 5.220153254 + -2.5 Combine like terms: 5.220153254 + -2.5 = 2.720153254 x = 2.720153254 Simplifying x = 2.720153254

Subproblem 2

x + 2.5 = -5.220153254 Simplifying x + 2.5 = -5.220153254 Reorder the terms: 2.5 + x = -5.220153254 Solving 2.5 + x = -5.220153254 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-2.5' to each side of the equation. 2.5 + -2.5 + x = -5.220153254 + -2.5 Combine like terms: 2.5 + -2.5 = 0.0 0.0 + x = -5.220153254 + -2.5 x = -5.220153254 + -2.5 Combine like terms: -5.220153254 + -2.5 = -7.720153254 x = -7.720153254 Simplifying x = -7.720153254

Solution

The solution to the problem is based on the solutions from the subproblems. x = {2.720153254, -7.720153254}

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