5x^2-15x-81=0

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Solution for 5x^2-15x-81=0 equation:


Simplifying
5x2 + -15x + -81 = 0

Reorder the terms:
-81 + -15x + 5x2 = 0

Solving
-81 + -15x + 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'.
-16.2 + -3x + x2 = 0

Move the constant term to the right:

Add '16.2' to each side of the equation.
-16.2 + -3x + 16.2 + x2 = 0 + 16.2

Reorder the terms:
-16.2 + 16.2 + -3x + x2 = 0 + 16.2

Combine like terms: -16.2 + 16.2 = 0.0
0.0 + -3x + x2 = 0 + 16.2
-3x + x2 = 0 + 16.2

Combine like terms: 0 + 16.2 = 16.2
-3x + x2 = 16.2

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 = 16.2 + 2.25

Reorder the terms:
2.25 + -3x + x2 = 16.2 + 2.25

Combine like terms: 16.2 + 2.25 = 18.45
2.25 + -3x + x2 = 18.45

Factor a perfect square on the left side:
(x + -1.5)(x + -1.5) = 18.45

Calculate the square root of the right side: 4.295346319

Break this problem into two subproblems by setting 
(x + -1.5) equal to 4.295346319 and -4.295346319.

Subproblem 1

x + -1.5 = 4.295346319 Simplifying x + -1.5 = 4.295346319 Reorder the terms: -1.5 + x = 4.295346319 Solving -1.5 + x = 4.295346319 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 = 4.295346319 + 1.5 Combine like terms: -1.5 + 1.5 = 0.0 0.0 + x = 4.295346319 + 1.5 x = 4.295346319 + 1.5 Combine like terms: 4.295346319 + 1.5 = 5.795346319 x = 5.795346319 Simplifying x = 5.795346319

Subproblem 2

x + -1.5 = -4.295346319 Simplifying x + -1.5 = -4.295346319 Reorder the terms: -1.5 + x = -4.295346319 Solving -1.5 + x = -4.295346319 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 = -4.295346319 + 1.5 Combine like terms: -1.5 + 1.5 = 0.0 0.0 + x = -4.295346319 + 1.5 x = -4.295346319 + 1.5 Combine like terms: -4.295346319 + 1.5 = -2.795346319 x = -2.795346319 Simplifying x = -2.795346319

Solution

The solution to the problem is based on the solutions from the subproblems. x = {5.795346319, -2.795346319}

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