8x^2+21x-28=0

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Solution for 8x^2+21x-28=0 equation:


Simplifying
8x2 + 21x + -28 = 0

Reorder the terms:
-28 + 21x + 8x2 = 0

Solving
-28 + 21x + 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'.
-3.5 + 2.625x + x2 = 0

Move the constant term to the right:

Add '3.5' to each side of the equation.
-3.5 + 2.625x + 3.5 + x2 = 0 + 3.5

Reorder the terms:
-3.5 + 3.5 + 2.625x + x2 = 0 + 3.5

Combine like terms: -3.5 + 3.5 = 0.0
0.0 + 2.625x + x2 = 0 + 3.5
2.625x + x2 = 0 + 3.5

Combine like terms: 0 + 3.5 = 3.5
2.625x + x2 = 3.5

The x term is 2.625x.  Take half its coefficient (1.3125).
Square it (1.72265625) and add it to both sides.

Add '1.72265625' to each side of the equation.
2.625x + 1.72265625 + x2 = 3.5 + 1.72265625

Reorder the terms:
1.72265625 + 2.625x + x2 = 3.5 + 1.72265625

Combine like terms: 3.5 + 1.72265625 = 5.22265625
1.72265625 + 2.625x + x2 = 5.22265625

Factor a perfect square on the left side:
(x + 1.3125)(x + 1.3125) = 5.22265625

Calculate the square root of the right side: 2.285313162

Break this problem into two subproblems by setting 
(x + 1.3125) equal to 2.285313162 and -2.285313162.

Subproblem 1

x + 1.3125 = 2.285313162 Simplifying x + 1.3125 = 2.285313162 Reorder the terms: 1.3125 + x = 2.285313162 Solving 1.3125 + x = 2.285313162 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-1.3125' to each side of the equation. 1.3125 + -1.3125 + x = 2.285313162 + -1.3125 Combine like terms: 1.3125 + -1.3125 = 0.0000 0.0000 + x = 2.285313162 + -1.3125 x = 2.285313162 + -1.3125 Combine like terms: 2.285313162 + -1.3125 = 0.972813162 x = 0.972813162 Simplifying x = 0.972813162

Subproblem 2

x + 1.3125 = -2.285313162 Simplifying x + 1.3125 = -2.285313162 Reorder the terms: 1.3125 + x = -2.285313162 Solving 1.3125 + x = -2.285313162 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-1.3125' to each side of the equation. 1.3125 + -1.3125 + x = -2.285313162 + -1.3125 Combine like terms: 1.3125 + -1.3125 = 0.0000 0.0000 + x = -2.285313162 + -1.3125 x = -2.285313162 + -1.3125 Combine like terms: -2.285313162 + -1.3125 = -3.597813162 x = -3.597813162 Simplifying x = -3.597813162

Solution

The solution to the problem is based on the solutions from the subproblems. x = {0.972813162, -3.597813162}

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