8x^2-21x-90=

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Solution for 8x^2-21x-90= equation:


Simplifying
8x2 + -21x + -90 = 0

Reorder the terms:
-90 + -21x + 8x2 = 0

Solving
-90 + -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'.
-11.25 + -2.625x + x2 = 0

Move the constant term to the right:

Add '11.25' to each side of the equation.
-11.25 + -2.625x + 11.25 + x2 = 0 + 11.25

Reorder the terms:
-11.25 + 11.25 + -2.625x + x2 = 0 + 11.25

Combine like terms: -11.25 + 11.25 = 0.00
0.00 + -2.625x + x2 = 0 + 11.25
-2.625x + x2 = 0 + 11.25

Combine like terms: 0 + 11.25 = 11.25
-2.625x + x2 = 11.25

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 = 11.25 + 1.72265625

Reorder the terms:
1.72265625 + -2.625x + x2 = 11.25 + 1.72265625

Combine like terms: 11.25 + 1.72265625 = 12.97265625
1.72265625 + -2.625x + x2 = 12.97265625

Factor a perfect square on the left side:
(x + -1.3125)(x + -1.3125) = 12.97265625

Calculate the square root of the right side: 3.601757384

Break this problem into two subproblems by setting 
(x + -1.3125) equal to 3.601757384 and -3.601757384.

Subproblem 1

x + -1.3125 = 3.601757384 Simplifying x + -1.3125 = 3.601757384 Reorder the terms: -1.3125 + x = 3.601757384 Solving -1.3125 + x = 3.601757384 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 = 3.601757384 + 1.3125 Combine like terms: -1.3125 + 1.3125 = 0.0000 0.0000 + x = 3.601757384 + 1.3125 x = 3.601757384 + 1.3125 Combine like terms: 3.601757384 + 1.3125 = 4.914257384 x = 4.914257384 Simplifying x = 4.914257384

Subproblem 2

x + -1.3125 = -3.601757384 Simplifying x + -1.3125 = -3.601757384 Reorder the terms: -1.3125 + x = -3.601757384 Solving -1.3125 + x = -3.601757384 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 = -3.601757384 + 1.3125 Combine like terms: -1.3125 + 1.3125 = 0.0000 0.0000 + x = -3.601757384 + 1.3125 x = -3.601757384 + 1.3125 Combine like terms: -3.601757384 + 1.3125 = -2.289257384 x = -2.289257384 Simplifying x = -2.289257384

Solution

The solution to the problem is based on the solutions from the subproblems. x = {4.914257384, -2.289257384}

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