x(21-2x)=21

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Solution for x(21-2x)=21 equation:


Simplifying
x(21 + -2x) = 21
(21 * x + -2x * x) = 21
(21x + -2x2) = 21

Solving
21x + -2x2 = 21

Solving for variable 'x'.

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

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

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

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

Move the constant term to the right:

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

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

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

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

The x term is -10.5x.  Take half its coefficient (-5.25).
Square it (27.5625) and add it to both sides.

Add '27.5625' to each side of the equation.
-10.5x + 27.5625 + x2 = -10.5 + 27.5625

Reorder the terms:
27.5625 + -10.5x + x2 = -10.5 + 27.5625

Combine like terms: -10.5 + 27.5625 = 17.0625
27.5625 + -10.5x + x2 = 17.0625

Factor a perfect square on the left side:
(x + -5.25)(x + -5.25) = 17.0625

Calculate the square root of the right side: 4.13067791

Break this problem into two subproblems by setting 
(x + -5.25) equal to 4.13067791 and -4.13067791.

Subproblem 1

x + -5.25 = 4.13067791 Simplifying x + -5.25 = 4.13067791 Reorder the terms: -5.25 + x = 4.13067791 Solving -5.25 + x = 4.13067791 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '5.25' to each side of the equation. -5.25 + 5.25 + x = 4.13067791 + 5.25 Combine like terms: -5.25 + 5.25 = 0.00 0.00 + x = 4.13067791 + 5.25 x = 4.13067791 + 5.25 Combine like terms: 4.13067791 + 5.25 = 9.38067791 x = 9.38067791 Simplifying x = 9.38067791

Subproblem 2

x + -5.25 = -4.13067791 Simplifying x + -5.25 = -4.13067791 Reorder the terms: -5.25 + x = -4.13067791 Solving -5.25 + x = -4.13067791 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '5.25' to each side of the equation. -5.25 + 5.25 + x = -4.13067791 + 5.25 Combine like terms: -5.25 + 5.25 = 0.00 0.00 + x = -4.13067791 + 5.25 x = -4.13067791 + 5.25 Combine like terms: -4.13067791 + 5.25 = 1.11932209 x = 1.11932209 Simplifying x = 1.11932209

Solution

The solution to the problem is based on the solutions from the subproblems. x = {9.38067791, 1.11932209}

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