-8x^2+5x+21=0

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Solution for -8x^2+5x+21=0 equation:


Simplifying
-8x2 + 5x + 21 = 0

Reorder the terms:
21 + 5x + -8x2 = 0

Solving
21 + 5x + -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 + -0.625x + x2 = 0

Move the constant term to the right:

Add '2.625' to each side of the equation.
-2.625 + -0.625x + 2.625 + x2 = 0 + 2.625

Reorder the terms:
-2.625 + 2.625 + -0.625x + x2 = 0 + 2.625

Combine like terms: -2.625 + 2.625 = 0.000
0.000 + -0.625x + x2 = 0 + 2.625
-0.625x + x2 = 0 + 2.625

Combine like terms: 0 + 2.625 = 2.625
-0.625x + x2 = 2.625

The x term is -0.625x.  Take half its coefficient (-0.3125).
Square it (0.09765625) and add it to both sides.

Add '0.09765625' to each side of the equation.
-0.625x + 0.09765625 + x2 = 2.625 + 0.09765625

Reorder the terms:
0.09765625 + -0.625x + x2 = 2.625 + 0.09765625

Combine like terms: 2.625 + 0.09765625 = 2.72265625
0.09765625 + -0.625x + x2 = 2.72265625

Factor a perfect square on the left side:
(x + -0.3125)(x + -0.3125) = 2.72265625

Calculate the square root of the right side: 1.650047348

Break this problem into two subproblems by setting 
(x + -0.3125) equal to 1.650047348 and -1.650047348.

Subproblem 1

x + -0.3125 = 1.650047348 Simplifying x + -0.3125 = 1.650047348 Reorder the terms: -0.3125 + x = 1.650047348 Solving -0.3125 + x = 1.650047348 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '0.3125' to each side of the equation. -0.3125 + 0.3125 + x = 1.650047348 + 0.3125 Combine like terms: -0.3125 + 0.3125 = 0.0000 0.0000 + x = 1.650047348 + 0.3125 x = 1.650047348 + 0.3125 Combine like terms: 1.650047348 + 0.3125 = 1.962547348 x = 1.962547348 Simplifying x = 1.962547348

Subproblem 2

x + -0.3125 = -1.650047348 Simplifying x + -0.3125 = -1.650047348 Reorder the terms: -0.3125 + x = -1.650047348 Solving -0.3125 + x = -1.650047348 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '0.3125' to each side of the equation. -0.3125 + 0.3125 + x = -1.650047348 + 0.3125 Combine like terms: -0.3125 + 0.3125 = 0.0000 0.0000 + x = -1.650047348 + 0.3125 x = -1.650047348 + 0.3125 Combine like terms: -1.650047348 + 0.3125 = -1.337547348 x = -1.337547348 Simplifying x = -1.337547348

Solution

The solution to the problem is based on the solutions from the subproblems. x = {1.962547348, -1.337547348}

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