5x^2-21x=18

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Solution for 5x^2-21x=18 equation:


Simplifying
5x2 + -21x = 18

Reorder the terms:
-21x + 5x2 = 18

Solving
-21x + 5x2 = 18

Solving for variable 'x'.

Reorder the terms:
-18 + -21x + 5x2 = 18 + -18

Combine like terms: 18 + -18 = 0
-18 + -21x + 5x2 = 0

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

Divide each side by '5'.
-3.6 + -4.2x + x2 = 0

Move the constant term to the right:

Add '3.6' to each side of the equation.
-3.6 + -4.2x + 3.6 + x2 = 0 + 3.6

Reorder the terms:
-3.6 + 3.6 + -4.2x + x2 = 0 + 3.6

Combine like terms: -3.6 + 3.6 = 0.0
0.0 + -4.2x + x2 = 0 + 3.6
-4.2x + x2 = 0 + 3.6

Combine like terms: 0 + 3.6 = 3.6
-4.2x + x2 = 3.6

The x term is -4.2x.  Take half its coefficient (-2.1).
Square it (4.41) and add it to both sides.

Add '4.41' to each side of the equation.
-4.2x + 4.41 + x2 = 3.6 + 4.41

Reorder the terms:
4.41 + -4.2x + x2 = 3.6 + 4.41

Combine like terms: 3.6 + 4.41 = 8.01
4.41 + -4.2x + x2 = 8.01

Factor a perfect square on the left side:
(x + -2.1)(x + -2.1) = 8.01

Calculate the square root of the right side: 2.83019434

Break this problem into two subproblems by setting 
(x + -2.1) equal to 2.83019434 and -2.83019434.

Subproblem 1

x + -2.1 = 2.83019434 Simplifying x + -2.1 = 2.83019434 Reorder the terms: -2.1 + x = 2.83019434 Solving -2.1 + x = 2.83019434 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '2.1' to each side of the equation. -2.1 + 2.1 + x = 2.83019434 + 2.1 Combine like terms: -2.1 + 2.1 = 0.0 0.0 + x = 2.83019434 + 2.1 x = 2.83019434 + 2.1 Combine like terms: 2.83019434 + 2.1 = 4.93019434 x = 4.93019434 Simplifying x = 4.93019434

Subproblem 2

x + -2.1 = -2.83019434 Simplifying x + -2.1 = -2.83019434 Reorder the terms: -2.1 + x = -2.83019434 Solving -2.1 + x = -2.83019434 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '2.1' to each side of the equation. -2.1 + 2.1 + x = -2.83019434 + 2.1 Combine like terms: -2.1 + 2.1 = 0.0 0.0 + x = -2.83019434 + 2.1 x = -2.83019434 + 2.1 Combine like terms: -2.83019434 + 2.1 = -0.73019434 x = -0.73019434 Simplifying x = -0.73019434

Solution

The solution to the problem is based on the solutions from the subproblems. x = {4.93019434, -0.73019434}

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