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 = 1.07251598 x = 1.07251598 Simplifying x = 1.07251598

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 = -2.44751598 x = -2.44751598 Simplifying x = -2.44751598

Solution

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

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