90=x*x+3x

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Solution for 90=x*x+3x equation:


Simplifying
90 = x * x + 3x

Multiply x * x
90 = x2 + 3x

Reorder the terms:
90 = 3x + x2

Solving
90 = 3x + x2

Solving for variable 'x'.

Reorder the terms:
90 + -3x + -1x2 = 3x + -3x + x2 + -1x2

Combine like terms: 3x + -3x = 0
90 + -3x + -1x2 = 0 + x2 + -1x2
90 + -3x + -1x2 = x2 + -1x2

Combine like terms: x2 + -1x2 = 0
90 + -3x + -1x2 = 0

Begin completing the square.  Divide all terms by
-1 the coefficient of the squared term: 

Divide each side by '-1'.
-90 + 3x + x2 = 0

Move the constant term to the right:

Add '90' to each side of the equation.
-90 + 3x + 90 + x2 = 0 + 90

Reorder the terms:
-90 + 90 + 3x + x2 = 0 + 90

Combine like terms: -90 + 90 = 0
0 + 3x + x2 = 0 + 90
3x + x2 = 0 + 90

Combine like terms: 0 + 90 = 90
3x + x2 = 90

The x term is 3x.  Take half its coefficient (1.5).
Square it (2.25) and add it to both sides.

Add '2.25' to each side of the equation.
3x + 2.25 + x2 = 90 + 2.25

Reorder the terms:
2.25 + 3x + x2 = 90 + 2.25

Combine like terms: 90 + 2.25 = 92.25
2.25 + 3x + x2 = 92.25

Factor a perfect square on the left side:
(x + 1.5)(x + 1.5) = 92.25

Calculate the square root of the right side: 9.604686356

Break this problem into two subproblems by setting 
(x + 1.5) equal to 9.604686356 and -9.604686356.

Subproblem 1

x + 1.5 = 9.604686356 Simplifying x + 1.5 = 9.604686356 Reorder the terms: 1.5 + x = 9.604686356 Solving 1.5 + x = 9.604686356 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-1.5' to each side of the equation. 1.5 + -1.5 + x = 9.604686356 + -1.5 Combine like terms: 1.5 + -1.5 = 0.0 0.0 + x = 9.604686356 + -1.5 x = 9.604686356 + -1.5 Combine like terms: 9.604686356 + -1.5 = 8.104686356 x = 8.104686356 Simplifying x = 8.104686356

Subproblem 2

x + 1.5 = -9.604686356 Simplifying x + 1.5 = -9.604686356 Reorder the terms: 1.5 + x = -9.604686356 Solving 1.5 + x = -9.604686356 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-1.5' to each side of the equation. 1.5 + -1.5 + x = -9.604686356 + -1.5 Combine like terms: 1.5 + -1.5 = 0.0 0.0 + x = -9.604686356 + -1.5 x = -9.604686356 + -1.5 Combine like terms: -9.604686356 + -1.5 = -11.104686356 x = -11.104686356 Simplifying x = -11.104686356

Solution

The solution to the problem is based on the solutions from the subproblems. x = {8.104686356, -11.104686356}

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