8x^2-11x-21=0

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Solution for 8x^2-11x-21=0 equation:


Simplifying
8x2 + -11x + -21 = 0

Reorder the terms:
-21 + -11x + 8x2 = 0

Solving
-21 + -11x + 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 + -1.375x + x2 = 0

Move the constant term to the right:

Add '2.625' to each side of the equation.
-2.625 + -1.375x + 2.625 + x2 = 0 + 2.625

Reorder the terms:
-2.625 + 2.625 + -1.375x + x2 = 0 + 2.625

Combine like terms: -2.625 + 2.625 = 0.000
0.000 + -1.375x + x2 = 0 + 2.625
-1.375x + x2 = 0 + 2.625

Combine like terms: 0 + 2.625 = 2.625
-1.375x + x2 = 2.625

The x term is -1.375x.  Take half its coefficient (-0.6875).
Square it (0.47265625) and add it to both sides.

Add '0.47265625' to each side of the equation.
-1.375x + 0.47265625 + x2 = 2.625 + 0.47265625

Reorder the terms:
0.47265625 + -1.375x + x2 = 2.625 + 0.47265625

Combine like terms: 2.625 + 0.47265625 = 3.09765625
0.47265625 + -1.375x + x2 = 3.09765625

Factor a perfect square on the left side:
(x + -0.6875)(x + -0.6875) = 3.09765625

Calculate the square root of the right side: 1.76001598

Break this problem into two subproblems by setting 
(x + -0.6875) equal to 1.76001598 and -1.76001598.

Subproblem 1

x + -0.6875 = 1.76001598 Simplifying x + -0.6875 = 1.76001598 Reorder the terms: -0.6875 + x = 1.76001598 Solving -0.6875 + x = 1.76001598 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '0.6875' to each side of the equation. -0.6875 + 0.6875 + x = 1.76001598 + 0.6875 Combine like terms: -0.6875 + 0.6875 = 0.0000 0.0000 + x = 1.76001598 + 0.6875 x = 1.76001598 + 0.6875 Combine like terms: 1.76001598 + 0.6875 = 2.44751598 x = 2.44751598 Simplifying x = 2.44751598

Subproblem 2

x + -0.6875 = -1.76001598 Simplifying x + -0.6875 = -1.76001598 Reorder the terms: -0.6875 + x = -1.76001598 Solving -0.6875 + x = -1.76001598 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '0.6875' to each side of the equation. -0.6875 + 0.6875 + x = -1.76001598 + 0.6875 Combine like terms: -0.6875 + 0.6875 = 0.0000 0.0000 + x = -1.76001598 + 0.6875 x = -1.76001598 + 0.6875 Combine like terms: -1.76001598 + 0.6875 = -1.07251598 x = -1.07251598 Simplifying x = -1.07251598

Solution

The solution to the problem is based on the solutions from the subproblems. x = {2.44751598, -1.07251598}

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