8x^2+21x-400=0

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Solution for 8x^2+21x-400=0 equation:


Simplifying
8x2 + 21x + -400 = 0

Reorder the terms:
-400 + 21x + 8x2 = 0

Solving
-400 + 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'.
-50 + 2.625x + x2 = 0

Move the constant term to the right:

Add '50' to each side of the equation.
-50 + 2.625x + 50 + x2 = 0 + 50

Reorder the terms:
-50 + 50 + 2.625x + x2 = 0 + 50

Combine like terms: -50 + 50 = 0
0 + 2.625x + x2 = 0 + 50
2.625x + x2 = 0 + 50

Combine like terms: 0 + 50 = 50
2.625x + x2 = 50

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 = 50 + 1.72265625

Reorder the terms:
1.72265625 + 2.625x + x2 = 50 + 1.72265625

Combine like terms: 50 + 1.72265625 = 51.72265625
1.72265625 + 2.625x + x2 = 51.72265625

Factor a perfect square on the left side:
(x + 1.3125)(x + 1.3125) = 51.72265625

Calculate the square root of the right side: 7.191846512

Break this problem into two subproblems by setting 
(x + 1.3125) equal to 7.191846512 and -7.191846512.

Subproblem 1

x + 1.3125 = 7.191846512 Simplifying x + 1.3125 = 7.191846512 Reorder the terms: 1.3125 + x = 7.191846512 Solving 1.3125 + x = 7.191846512 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 = 7.191846512 + -1.3125 Combine like terms: 1.3125 + -1.3125 = 0.0000 0.0000 + x = 7.191846512 + -1.3125 x = 7.191846512 + -1.3125 Combine like terms: 7.191846512 + -1.3125 = 5.879346512 x = 5.879346512 Simplifying x = 5.879346512

Subproblem 2

x + 1.3125 = -7.191846512 Simplifying x + 1.3125 = -7.191846512 Reorder the terms: 1.3125 + x = -7.191846512 Solving 1.3125 + x = -7.191846512 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 = -7.191846512 + -1.3125 Combine like terms: 1.3125 + -1.3125 = 0.0000 0.0000 + x = -7.191846512 + -1.3125 x = -7.191846512 + -1.3125 Combine like terms: -7.191846512 + -1.3125 = -8.504346512 x = -8.504346512 Simplifying x = -8.504346512

Solution

The solution to the problem is based on the solutions from the subproblems. x = {5.879346512, -8.504346512}

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