5x^2+2x-21=0

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Solution for 5x^2+2x-21=0 equation:


Simplifying
5x2 + 2x + -21 = 0

Reorder the terms:
-21 + 2x + 5x2 = 0

Solving
-21 + 2x + 5x2 = 0

Solving for variable 'x'.

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

Divide each side by '5'.
-4.2 + 0.4x + x2 = 0

Move the constant term to the right:

Add '4.2' to each side of the equation.
-4.2 + 0.4x + 4.2 + x2 = 0 + 4.2

Reorder the terms:
-4.2 + 4.2 + 0.4x + x2 = 0 + 4.2

Combine like terms: -4.2 + 4.2 = 0.0
0.0 + 0.4x + x2 = 0 + 4.2
0.4x + x2 = 0 + 4.2

Combine like terms: 0 + 4.2 = 4.2
0.4x + x2 = 4.2

The x term is 0.4x.  Take half its coefficient (0.2).
Square it (0.04) and add it to both sides.

Add '0.04' to each side of the equation.
0.4x + 0.04 + x2 = 4.2 + 0.04

Reorder the terms:
0.04 + 0.4x + x2 = 4.2 + 0.04

Combine like terms: 4.2 + 0.04 = 4.24
0.04 + 0.4x + x2 = 4.24

Factor a perfect square on the left side:
(x + 0.2)(x + 0.2) = 4.24

Calculate the square root of the right side: 2.059126028

Break this problem into two subproblems by setting 
(x + 0.2) equal to 2.059126028 and -2.059126028.

Subproblem 1

x + 0.2 = 2.059126028 Simplifying x + 0.2 = 2.059126028 Reorder the terms: 0.2 + x = 2.059126028 Solving 0.2 + x = 2.059126028 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-0.2' to each side of the equation. 0.2 + -0.2 + x = 2.059126028 + -0.2 Combine like terms: 0.2 + -0.2 = 0.0 0.0 + x = 2.059126028 + -0.2 x = 2.059126028 + -0.2 Combine like terms: 2.059126028 + -0.2 = 1.859126028 x = 1.859126028 Simplifying x = 1.859126028

Subproblem 2

x + 0.2 = -2.059126028 Simplifying x + 0.2 = -2.059126028 Reorder the terms: 0.2 + x = -2.059126028 Solving 0.2 + x = -2.059126028 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-0.2' to each side of the equation. 0.2 + -0.2 + x = -2.059126028 + -0.2 Combine like terms: 0.2 + -0.2 = 0.0 0.0 + x = -2.059126028 + -0.2 x = -2.059126028 + -0.2 Combine like terms: -2.059126028 + -0.2 = -2.259126028 x = -2.259126028 Simplifying x = -2.259126028

Solution

The solution to the problem is based on the solutions from the subproblems. x = {1.859126028, -2.259126028}

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