x(4x+15)-21=0

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Solution for x(4x+15)-21=0 equation:


Simplifying
x(4x + 15) + -21 = 0

Reorder the terms:
x(15 + 4x) + -21 = 0
(15 * x + 4x * x) + -21 = 0
(15x + 4x2) + -21 = 0

Reorder the terms:
-21 + 15x + 4x2 = 0

Solving
-21 + 15x + 4x2 = 0

Solving for variable 'x'.

Begin completing the square.  Divide all terms by
4 the coefficient of the squared term: 

Divide each side by '4'.
-5.25 + 3.75x + x2 = 0

Move the constant term to the right:

Add '5.25' to each side of the equation.
-5.25 + 3.75x + 5.25 + x2 = 0 + 5.25

Reorder the terms:
-5.25 + 5.25 + 3.75x + x2 = 0 + 5.25

Combine like terms: -5.25 + 5.25 = 0.00
0.00 + 3.75x + x2 = 0 + 5.25
3.75x + x2 = 0 + 5.25

Combine like terms: 0 + 5.25 = 5.25
3.75x + x2 = 5.25

The x term is 3.75x.  Take half its coefficient (1.875).
Square it (3.515625) and add it to both sides.

Add '3.515625' to each side of the equation.
3.75x + 3.515625 + x2 = 5.25 + 3.515625

Reorder the terms:
3.515625 + 3.75x + x2 = 5.25 + 3.515625

Combine like terms: 5.25 + 3.515625 = 8.765625
3.515625 + 3.75x + x2 = 8.765625

Factor a perfect square on the left side:
(x + 1.875)(x + 1.875) = 8.765625

Calculate the square root of the right side: 2.960679821

Break this problem into two subproblems by setting 
(x + 1.875) equal to 2.960679821 and -2.960679821.

Subproblem 1

x + 1.875 = 2.960679821 Simplifying x + 1.875 = 2.960679821 Reorder the terms: 1.875 + x = 2.960679821 Solving 1.875 + x = 2.960679821 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-1.875' to each side of the equation. 1.875 + -1.875 + x = 2.960679821 + -1.875 Combine like terms: 1.875 + -1.875 = 0.000 0.000 + x = 2.960679821 + -1.875 x = 2.960679821 + -1.875 Combine like terms: 2.960679821 + -1.875 = 1.085679821 x = 1.085679821 Simplifying x = 1.085679821

Subproblem 2

x + 1.875 = -2.960679821 Simplifying x + 1.875 = -2.960679821 Reorder the terms: 1.875 + x = -2.960679821 Solving 1.875 + x = -2.960679821 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-1.875' to each side of the equation. 1.875 + -1.875 + x = -2.960679821 + -1.875 Combine like terms: 1.875 + -1.875 = 0.000 0.000 + x = -2.960679821 + -1.875 x = -2.960679821 + -1.875 Combine like terms: -2.960679821 + -1.875 = -4.835679821 x = -4.835679821 Simplifying x = -4.835679821

Solution

The solution to the problem is based on the solutions from the subproblems. x = {1.085679821, -4.835679821}

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