x^4-16x^2-81=0

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Solution for x^4-16x^2-81=0 equation:


Simplifying
x4 + -16x2 + -81 = 0

Reorder the terms:
-81 + -16x2 + x4 = 0

Solving
-81 + -16x2 + x4 = 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 + -16x2 + 81 + x4 = 0 + 81

Reorder the terms:
-81 + 81 + -16x2 + x4 = 0 + 81

Combine like terms: -81 + 81 = 0
0 + -16x2 + x4 = 0 + 81
-16x2 + x4 = 0 + 81

Combine like terms: 0 + 81 = 81
-16x2 + x4 = 81

The x term is -16x2.  Take half its coefficient (-8).
Square it (64) and add it to both sides.

Add '64' to each side of the equation.
-16x2 + 64 + x4 = 81 + 64

Reorder the terms:
64 + -16x2 + x4 = 81 + 64

Combine like terms: 81 + 64 = 145
64 + -16x2 + x4 = 145

Factor a perfect square on the left side:
(x2 + -8)(x2 + -8) = 145

Calculate the square root of the right side: 12.041594579

Break this problem into two subproblems by setting 
(x2 + -8) equal to 12.041594579 and -12.041594579.

Subproblem 1

x2 + -8 = 12.041594579 Simplifying x2 + -8 = 12.041594579 Reorder the terms: -8 + x2 = 12.041594579 Solving -8 + x2 = 12.041594579 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '8' to each side of the equation. -8 + 8 + x2 = 12.041594579 + 8 Combine like terms: -8 + 8 = 0 0 + x2 = 12.041594579 + 8 x2 = 12.041594579 + 8 Combine like terms: 12.041594579 + 8 = 20.041594579 x2 = 20.041594579 Simplifying x2 = 20.041594579 Take the square root of each side: x = {-4.476783955, 4.476783955}

Subproblem 2

x2 + -8 = -12.041594579 Simplifying x2 + -8 = -12.041594579 Reorder the terms: -8 + x2 = -12.041594579 Solving -8 + x2 = -12.041594579 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '8' to each side of the equation. -8 + 8 + x2 = -12.041594579 + 8 Combine like terms: -8 + 8 = 0 0 + x2 = -12.041594579 + 8 x2 = -12.041594579 + 8 Combine like terms: -12.041594579 + 8 = -4.041594579 x2 = -4.041594579 Simplifying x2 = -4.041594579 Reorder the terms: 4.041594579 + x2 = -4.041594579 + 4.041594579 Combine like terms: -4.041594579 + 4.041594579 = 0.000000000 4.041594579 + x2 = 0.000000000 The solution to this equation could not be determined.This subproblem is being ignored because a solution could not be determined. The solution to this equation could not be determined.

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