3x^2-8x-81=0

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Solution for 3x^2-8x-81=0 equation:


Simplifying
3x2 + -8x + -81 = 0

Reorder the terms:
-81 + -8x + 3x2 = 0

Solving
-81 + -8x + 3x2 = 0

Solving for variable 'x'.

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

Divide each side by '3'.
-27 + -2.666666667x + x2 = 0

Move the constant term to the right:

Add '27' to each side of the equation.
-27 + -2.666666667x + 27 + x2 = 0 + 27

Reorder the terms:
-27 + 27 + -2.666666667x + x2 = 0 + 27

Combine like terms: -27 + 27 = 0
0 + -2.666666667x + x2 = 0 + 27
-2.666666667x + x2 = 0 + 27

Combine like terms: 0 + 27 = 27
-2.666666667x + x2 = 27

The x term is -2.666666667x.  Take half its coefficient (-1.333333334).
Square it (1.777777780) and add it to both sides.

Add '1.777777780' to each side of the equation.
-2.666666667x + 1.777777780 + x2 = 27 + 1.777777780

Reorder the terms:
1.777777780 + -2.666666667x + x2 = 27 + 1.777777780

Combine like terms: 27 + 1.777777780 = 28.77777778
1.777777780 + -2.666666667x + x2 = 28.77777778

Factor a perfect square on the left side:
(x + -1.333333334)(x + -1.333333334) = 28.77777778

Calculate the square root of the right side: 5.364492313

Break this problem into two subproblems by setting 
(x + -1.333333334) equal to 5.364492313 and -5.364492313.

Subproblem 1

x + -1.333333334 = 5.364492313 Simplifying x + -1.333333334 = 5.364492313 Reorder the terms: -1.333333334 + x = 5.364492313 Solving -1.333333334 + x = 5.364492313 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '1.333333334' to each side of the equation. -1.333333334 + 1.333333334 + x = 5.364492313 + 1.333333334 Combine like terms: -1.333333334 + 1.333333334 = 0.000000000 0.000000000 + x = 5.364492313 + 1.333333334 x = 5.364492313 + 1.333333334 Combine like terms: 5.364492313 + 1.333333334 = 6.697825647 x = 6.697825647 Simplifying x = 6.697825647

Subproblem 2

x + -1.333333334 = -5.364492313 Simplifying x + -1.333333334 = -5.364492313 Reorder the terms: -1.333333334 + x = -5.364492313 Solving -1.333333334 + x = -5.364492313 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '1.333333334' to each side of the equation. -1.333333334 + 1.333333334 + x = -5.364492313 + 1.333333334 Combine like terms: -1.333333334 + 1.333333334 = 0.000000000 0.000000000 + x = -5.364492313 + 1.333333334 x = -5.364492313 + 1.333333334 Combine like terms: -5.364492313 + 1.333333334 = -4.031158979 x = -4.031158979 Simplifying x = -4.031158979

Solution

The solution to the problem is based on the solutions from the subproblems. x = {6.697825647, -4.031158979}

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