5*x^2+21*x=55

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Solution for 5*x^2+21*x=55 equation:


Simplifying
5x2 + 21x = 55

Reorder the terms:
21x + 5x2 = 55

Solving
21x + 5x2 = 55

Solving for variable 'x'.

Reorder the terms:
-55 + 21x + 5x2 = 55 + -55

Combine like terms: 55 + -55 = 0
-55 + 21x + 5x2 = 0

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

Divide each side by '5'.
-11 + 4.2x + x2 = 0

Move the constant term to the right:

Add '11' to each side of the equation.
-11 + 4.2x + 11 + x2 = 0 + 11

Reorder the terms:
-11 + 11 + 4.2x + x2 = 0 + 11

Combine like terms: -11 + 11 = 0
0 + 4.2x + x2 = 0 + 11
4.2x + x2 = 0 + 11

Combine like terms: 0 + 11 = 11
4.2x + x2 = 11

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 = 11 + 4.41

Reorder the terms:
4.41 + 4.2x + x2 = 11 + 4.41

Combine like terms: 11 + 4.41 = 15.41
4.41 + 4.2x + x2 = 15.41

Factor a perfect square on the left side:
(x + 2.1)(x + 2.1) = 15.41

Calculate the square root of the right side: 3.925557285

Break this problem into two subproblems by setting 
(x + 2.1) equal to 3.925557285 and -3.925557285.

Subproblem 1

x + 2.1 = 3.925557285 Simplifying x + 2.1 = 3.925557285 Reorder the terms: 2.1 + x = 3.925557285 Solving 2.1 + x = 3.925557285 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 = 3.925557285 + -2.1 Combine like terms: 2.1 + -2.1 = 0.0 0.0 + x = 3.925557285 + -2.1 x = 3.925557285 + -2.1 Combine like terms: 3.925557285 + -2.1 = 1.825557285 x = 1.825557285 Simplifying x = 1.825557285

Subproblem 2

x + 2.1 = -3.925557285 Simplifying x + 2.1 = -3.925557285 Reorder the terms: 2.1 + x = -3.925557285 Solving 2.1 + x = -3.925557285 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 = -3.925557285 + -2.1 Combine like terms: 2.1 + -2.1 = 0.0 0.0 + x = -3.925557285 + -2.1 x = -3.925557285 + -2.1 Combine like terms: -3.925557285 + -2.1 = -6.025557285 x = -6.025557285 Simplifying x = -6.025557285

Solution

The solution to the problem is based on the solutions from the subproblems. x = {1.825557285, -6.025557285}

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