8x^2-20x-21=0

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Solution for 8x^2-20x-21=0 equation:


Simplifying
8x2 + -20x + -21 = 0

Reorder the terms:
-21 + -20x + 8x2 = 0

Solving
-21 + -20x + 8x2 = 0

Solving for variable 'x'.

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

Divide each side by '8'.
-2.625 + -2.5x + x2 = 0

Move the constant term to the right:

Add '2.625' to each side of the equation.
-2.625 + -2.5x + 2.625 + x2 = 0 + 2.625

Reorder the terms:
-2.625 + 2.625 + -2.5x + x2 = 0 + 2.625

Combine like terms: -2.625 + 2.625 = 0.000
0.000 + -2.5x + x2 = 0 + 2.625
-2.5x + x2 = 0 + 2.625

Combine like terms: 0 + 2.625 = 2.625
-2.5x + x2 = 2.625

The x term is -2.5x.  Take half its coefficient (-1.25).
Square it (1.5625) and add it to both sides.

Add '1.5625' to each side of the equation.
-2.5x + 1.5625 + x2 = 2.625 + 1.5625

Reorder the terms:
1.5625 + -2.5x + x2 = 2.625 + 1.5625

Combine like terms: 2.625 + 1.5625 = 4.1875
1.5625 + -2.5x + x2 = 4.1875

Factor a perfect square on the left side:
(x + -1.25)(x + -1.25) = 4.1875

Calculate the square root of the right side: 2.046338193

Break this problem into two subproblems by setting 
(x + -1.25) equal to 2.046338193 and -2.046338193.

Subproblem 1

x + -1.25 = 2.046338193 Simplifying x + -1.25 = 2.046338193 Reorder the terms: -1.25 + x = 2.046338193 Solving -1.25 + x = 2.046338193 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '1.25' to each side of the equation. -1.25 + 1.25 + x = 2.046338193 + 1.25 Combine like terms: -1.25 + 1.25 = 0.00 0.00 + x = 2.046338193 + 1.25 x = 2.046338193 + 1.25 Combine like terms: 2.046338193 + 1.25 = 3.296338193 x = 3.296338193 Simplifying x = 3.296338193

Subproblem 2

x + -1.25 = -2.046338193 Simplifying x + -1.25 = -2.046338193 Reorder the terms: -1.25 + x = -2.046338193 Solving -1.25 + x = -2.046338193 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '1.25' to each side of the equation. -1.25 + 1.25 + x = -2.046338193 + 1.25 Combine like terms: -1.25 + 1.25 = 0.00 0.00 + x = -2.046338193 + 1.25 x = -2.046338193 + 1.25 Combine like terms: -2.046338193 + 1.25 = -0.796338193 x = -0.796338193 Simplifying x = -0.796338193

Solution

The solution to the problem is based on the solutions from the subproblems. x = {3.296338193, -0.796338193}

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