5x^2+7x=21

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Solution for 5x^2+7x=21 equation:


Simplifying
5x2 + 7x = 21

Reorder the terms:
7x + 5x2 = 21

Solving
7x + 5x2 = 21

Solving for variable 'x'.

Reorder the terms:
-21 + 7x + 5x2 = 21 + -21

Combine like terms: 21 + -21 = 0
-21 + 7x + 5x2 = 0

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

Divide each side by '5'.
-4.2 + 1.4x + x2 = 0

Move the constant term to the right:

Add '4.2' to each side of the equation.
-4.2 + 1.4x + 4.2 + x2 = 0 + 4.2

Reorder the terms:
-4.2 + 4.2 + 1.4x + x2 = 0 + 4.2

Combine like terms: -4.2 + 4.2 = 0.0
0.0 + 1.4x + x2 = 0 + 4.2
1.4x + x2 = 0 + 4.2

Combine like terms: 0 + 4.2 = 4.2
1.4x + x2 = 4.2

The x term is 1.4x.  Take half its coefficient (0.7).
Square it (0.49) and add it to both sides.

Add '0.49' to each side of the equation.
1.4x + 0.49 + x2 = 4.2 + 0.49

Reorder the terms:
0.49 + 1.4x + x2 = 4.2 + 0.49

Combine like terms: 4.2 + 0.49 = 4.69
0.49 + 1.4x + x2 = 4.69

Factor a perfect square on the left side:
(x + 0.7)(x + 0.7) = 4.69

Calculate the square root of the right side: 2.165640783

Break this problem into two subproblems by setting 
(x + 0.7) equal to 2.165640783 and -2.165640783.

Subproblem 1

x + 0.7 = 2.165640783 Simplifying x + 0.7 = 2.165640783 Reorder the terms: 0.7 + x = 2.165640783 Solving 0.7 + x = 2.165640783 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-0.7' to each side of the equation. 0.7 + -0.7 + x = 2.165640783 + -0.7 Combine like terms: 0.7 + -0.7 = 0.0 0.0 + x = 2.165640783 + -0.7 x = 2.165640783 + -0.7 Combine like terms: 2.165640783 + -0.7 = 1.465640783 x = 1.465640783 Simplifying x = 1.465640783

Subproblem 2

x + 0.7 = -2.165640783 Simplifying x + 0.7 = -2.165640783 Reorder the terms: 0.7 + x = -2.165640783 Solving 0.7 + x = -2.165640783 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-0.7' to each side of the equation. 0.7 + -0.7 + x = -2.165640783 + -0.7 Combine like terms: 0.7 + -0.7 = 0.0 0.0 + x = -2.165640783 + -0.7 x = -2.165640783 + -0.7 Combine like terms: -2.165640783 + -0.7 = -2.865640783 x = -2.865640783 Simplifying x = -2.865640783

Solution

The solution to the problem is based on the solutions from the subproblems. x = {1.465640783, -2.865640783}

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