x^2+15x-21=0

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Solution for x^2+15x-21=0 equation:


Simplifying
x2 + 15x + -21 = 0

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

Solving
-21 + 15x + x2 = 0

Solving for variable 'x'.

Begin completing the square.

Move the constant term to the right:

Add '21' to each side of the equation.
-21 + 15x + 21 + x2 = 0 + 21

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

Combine like terms: -21 + 21 = 0
0 + 15x + x2 = 0 + 21
15x + x2 = 0 + 21

Combine like terms: 0 + 21 = 21
15x + x2 = 21

The x term is 15x.  Take half its coefficient (7.5).
Square it (56.25) and add it to both sides.

Add '56.25' to each side of the equation.
15x + 56.25 + x2 = 21 + 56.25

Reorder the terms:
56.25 + 15x + x2 = 21 + 56.25

Combine like terms: 21 + 56.25 = 77.25
56.25 + 15x + x2 = 77.25

Factor a perfect square on the left side:
(x + 7.5)(x + 7.5) = 77.25

Calculate the square root of the right side: 8.789197916

Break this problem into two subproblems by setting 
(x + 7.5) equal to 8.789197916 and -8.789197916.

Subproblem 1

x + 7.5 = 8.789197916 Simplifying x + 7.5 = 8.789197916 Reorder the terms: 7.5 + x = 8.789197916 Solving 7.5 + x = 8.789197916 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-7.5' to each side of the equation. 7.5 + -7.5 + x = 8.789197916 + -7.5 Combine like terms: 7.5 + -7.5 = 0.0 0.0 + x = 8.789197916 + -7.5 x = 8.789197916 + -7.5 Combine like terms: 8.789197916 + -7.5 = 1.289197916 x = 1.289197916 Simplifying x = 1.289197916

Subproblem 2

x + 7.5 = -8.789197916 Simplifying x + 7.5 = -8.789197916 Reorder the terms: 7.5 + x = -8.789197916 Solving 7.5 + x = -8.789197916 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-7.5' to each side of the equation. 7.5 + -7.5 + x = -8.789197916 + -7.5 Combine like terms: 7.5 + -7.5 = 0.0 0.0 + x = -8.789197916 + -7.5 x = -8.789197916 + -7.5 Combine like terms: -8.789197916 + -7.5 = -16.289197916 x = -16.289197916 Simplifying x = -16.289197916

Solution

The solution to the problem is based on the solutions from the subproblems. x = {1.289197916, -16.289197916}

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