5x^2-21=12x

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Solution for 5x^2-21=12x equation:


Simplifying
5x2 + -21 = 12x

Reorder the terms:
-21 + 5x2 = 12x

Solving
-21 + 5x2 = 12x

Solving for variable 'x'.

Reorder the terms:
-21 + -12x + 5x2 = 12x + -12x

Combine like terms: 12x + -12x = 0
-21 + -12x + 5x2 = 0

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

Divide each side by '5'.
-4.2 + -2.4x + x2 = 0

Move the constant term to the right:

Add '4.2' to each side of the equation.
-4.2 + -2.4x + 4.2 + x2 = 0 + 4.2

Reorder the terms:
-4.2 + 4.2 + -2.4x + x2 = 0 + 4.2

Combine like terms: -4.2 + 4.2 = 0.0
0.0 + -2.4x + x2 = 0 + 4.2
-2.4x + x2 = 0 + 4.2

Combine like terms: 0 + 4.2 = 4.2
-2.4x + x2 = 4.2

The x term is -2.4x.  Take half its coefficient (-1.2).
Square it (1.44) and add it to both sides.

Add '1.44' to each side of the equation.
-2.4x + 1.44 + x2 = 4.2 + 1.44

Reorder the terms:
1.44 + -2.4x + x2 = 4.2 + 1.44

Combine like terms: 4.2 + 1.44 = 5.64
1.44 + -2.4x + x2 = 5.64

Factor a perfect square on the left side:
(x + -1.2)(x + -1.2) = 5.64

Calculate the square root of the right side: 2.374868417

Break this problem into two subproblems by setting 
(x + -1.2) equal to 2.374868417 and -2.374868417.

Subproblem 1

x + -1.2 = 2.374868417 Simplifying x + -1.2 = 2.374868417 Reorder the terms: -1.2 + x = 2.374868417 Solving -1.2 + x = 2.374868417 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '1.2' to each side of the equation. -1.2 + 1.2 + x = 2.374868417 + 1.2 Combine like terms: -1.2 + 1.2 = 0.0 0.0 + x = 2.374868417 + 1.2 x = 2.374868417 + 1.2 Combine like terms: 2.374868417 + 1.2 = 3.574868417 x = 3.574868417 Simplifying x = 3.574868417

Subproblem 2

x + -1.2 = -2.374868417 Simplifying x + -1.2 = -2.374868417 Reorder the terms: -1.2 + x = -2.374868417 Solving -1.2 + x = -2.374868417 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '1.2' to each side of the equation. -1.2 + 1.2 + x = -2.374868417 + 1.2 Combine like terms: -1.2 + 1.2 = 0.0 0.0 + x = -2.374868417 + 1.2 x = -2.374868417 + 1.2 Combine like terms: -2.374868417 + 1.2 = -1.174868417 x = -1.174868417 Simplifying x = -1.174868417

Solution

The solution to the problem is based on the solutions from the subproblems. x = {3.574868417, -1.174868417}

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