x^2-4x-81=0

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Solution for x^2-4x-81=0 equation:


Simplifying
x2 + -4x + -81 = 0

Reorder the terms:
-81 + -4x + x2 = 0

Solving
-81 + -4x + x2 = 0

Solving for variable 'x'.

Begin completing the square.

Move the constant term to the right:

Add '81' to each side of the equation.
-81 + -4x + 81 + x2 = 0 + 81

Reorder the terms:
-81 + 81 + -4x + x2 = 0 + 81

Combine like terms: -81 + 81 = 0
0 + -4x + x2 = 0 + 81
-4x + x2 = 0 + 81

Combine like terms: 0 + 81 = 81
-4x + x2 = 81

The x term is -4x.  Take half its coefficient (-2).
Square it (4) and add it to both sides.

Add '4' to each side of the equation.
-4x + 4 + x2 = 81 + 4

Reorder the terms:
4 + -4x + x2 = 81 + 4

Combine like terms: 81 + 4 = 85
4 + -4x + x2 = 85

Factor a perfect square on the left side:
(x + -2)(x + -2) = 85

Calculate the square root of the right side: 9.219544457

Break this problem into two subproblems by setting 
(x + -2) equal to 9.219544457 and -9.219544457.

Subproblem 1

x + -2 = 9.219544457 Simplifying x + -2 = 9.219544457 Reorder the terms: -2 + x = 9.219544457 Solving -2 + x = 9.219544457 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '2' to each side of the equation. -2 + 2 + x = 9.219544457 + 2 Combine like terms: -2 + 2 = 0 0 + x = 9.219544457 + 2 x = 9.219544457 + 2 Combine like terms: 9.219544457 + 2 = 11.219544457 x = 11.219544457 Simplifying x = 11.219544457

Subproblem 2

x + -2 = -9.219544457 Simplifying x + -2 = -9.219544457 Reorder the terms: -2 + x = -9.219544457 Solving -2 + x = -9.219544457 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '2' to each side of the equation. -2 + 2 + x = -9.219544457 + 2 Combine like terms: -2 + 2 = 0 0 + x = -9.219544457 + 2 x = -9.219544457 + 2 Combine like terms: -9.219544457 + 2 = -7.219544457 x = -7.219544457 Simplifying x = -7.219544457

Solution

The solution to the problem is based on the solutions from the subproblems. x = {11.219544457, -7.219544457}

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