2x^2-81x-21=0

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Solution for 2x^2-81x-21=0 equation:


Simplifying
2x2 + -81x + -21 = 0

Reorder the terms:
-21 + -81x + 2x2 = 0

Solving
-21 + -81x + 2x2 = 0

Solving for variable 'x'.

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

Divide each side by '2'.
-10.5 + -40.5x + x2 = 0

Move the constant term to the right:

Add '10.5' to each side of the equation.
-10.5 + -40.5x + 10.5 + x2 = 0 + 10.5

Reorder the terms:
-10.5 + 10.5 + -40.5x + x2 = 0 + 10.5

Combine like terms: -10.5 + 10.5 = 0.0
0.0 + -40.5x + x2 = 0 + 10.5
-40.5x + x2 = 0 + 10.5

Combine like terms: 0 + 10.5 = 10.5
-40.5x + x2 = 10.5

The x term is -40.5x.  Take half its coefficient (-20.25).
Square it (410.0625) and add it to both sides.

Add '410.0625' to each side of the equation.
-40.5x + 410.0625 + x2 = 10.5 + 410.0625

Reorder the terms:
410.0625 + -40.5x + x2 = 10.5 + 410.0625

Combine like terms: 10.5 + 410.0625 = 420.5625
410.0625 + -40.5x + x2 = 420.5625

Factor a perfect square on the left side:
(x + -20.25)(x + -20.25) = 420.5625

Calculate the square root of the right side: 20.507620535

Break this problem into two subproblems by setting 
(x + -20.25) equal to 20.507620535 and -20.507620535.

Subproblem 1

x + -20.25 = 20.507620535 Simplifying x + -20.25 = 20.507620535 Reorder the terms: -20.25 + x = 20.507620535 Solving -20.25 + x = 20.507620535 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '20.25' to each side of the equation. -20.25 + 20.25 + x = 20.507620535 + 20.25 Combine like terms: -20.25 + 20.25 = 0.00 0.00 + x = 20.507620535 + 20.25 x = 20.507620535 + 20.25 Combine like terms: 20.507620535 + 20.25 = 40.757620535 x = 40.757620535 Simplifying x = 40.757620535

Subproblem 2

x + -20.25 = -20.507620535 Simplifying x + -20.25 = -20.507620535 Reorder the terms: -20.25 + x = -20.507620535 Solving -20.25 + x = -20.507620535 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '20.25' to each side of the equation. -20.25 + 20.25 + x = -20.507620535 + 20.25 Combine like terms: -20.25 + 20.25 = 0.00 0.00 + x = -20.507620535 + 20.25 x = -20.507620535 + 20.25 Combine like terms: -20.507620535 + 20.25 = -0.257620535 x = -0.257620535 Simplifying x = -0.257620535

Solution

The solution to the problem is based on the solutions from the subproblems. x = {40.757620535, -0.257620535}

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