8x^2+21x+4=0

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Solution for 8x^2+21x+4=0 equation:


Simplifying
8x2 + 21x + 4 = 0

Reorder the terms:
4 + 21x + 8x2 = 0

Solving
4 + 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'.
0.5 + 2.625x + x2 = 0

Move the constant term to the right:

Add '-0.5' to each side of the equation.
0.5 + 2.625x + -0.5 + x2 = 0 + -0.5

Reorder the terms:
0.5 + -0.5 + 2.625x + x2 = 0 + -0.5

Combine like terms: 0.5 + -0.5 = 0.0
0.0 + 2.625x + x2 = 0 + -0.5
2.625x + x2 = 0 + -0.5

Combine like terms: 0 + -0.5 = -0.5
2.625x + x2 = -0.5

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 = -0.5 + 1.72265625

Reorder the terms:
1.72265625 + 2.625x + x2 = -0.5 + 1.72265625

Combine like terms: -0.5 + 1.72265625 = 1.22265625
1.72265625 + 2.625x + x2 = 1.22265625

Factor a perfect square on the left side:
(x + 1.3125)(x + 1.3125) = 1.22265625

Calculate the square root of the right side: 1.105737876

Break this problem into two subproblems by setting 
(x + 1.3125) equal to 1.105737876 and -1.105737876.

Subproblem 1

x + 1.3125 = 1.105737876 Simplifying x + 1.3125 = 1.105737876 Reorder the terms: 1.3125 + x = 1.105737876 Solving 1.3125 + x = 1.105737876 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 = 1.105737876 + -1.3125 Combine like terms: 1.3125 + -1.3125 = 0.0000 0.0000 + x = 1.105737876 + -1.3125 x = 1.105737876 + -1.3125 Combine like terms: 1.105737876 + -1.3125 = -0.206762124 x = -0.206762124 Simplifying x = -0.206762124

Subproblem 2

x + 1.3125 = -1.105737876 Simplifying x + 1.3125 = -1.105737876 Reorder the terms: 1.3125 + x = -1.105737876 Solving 1.3125 + x = -1.105737876 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 = -1.105737876 + -1.3125 Combine like terms: 1.3125 + -1.3125 = 0.0000 0.0000 + x = -1.105737876 + -1.3125 x = -1.105737876 + -1.3125 Combine like terms: -1.105737876 + -1.3125 = -2.418237876 x = -2.418237876 Simplifying x = -2.418237876

Solution

The solution to the problem is based on the solutions from the subproblems. x = {-0.206762124, -2.418237876}

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