x^2-10x-21=0

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Solution for x^2-10x-21=0 equation:


Simplifying
x2 + -10x + -21 = 0

Reorder the terms:
-21 + -10x + x2 = 0

Solving
-21 + -10x + 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 + -10x + 21 + x2 = 0 + 21

Reorder the terms:
-21 + 21 + -10x + x2 = 0 + 21

Combine like terms: -21 + 21 = 0
0 + -10x + x2 = 0 + 21
-10x + x2 = 0 + 21

Combine like terms: 0 + 21 = 21
-10x + x2 = 21

The x term is -10x.  Take half its coefficient (-5).
Square it (25) and add it to both sides.

Add '25' to each side of the equation.
-10x + 25 + x2 = 21 + 25

Reorder the terms:
25 + -10x + x2 = 21 + 25

Combine like terms: 21 + 25 = 46
25 + -10x + x2 = 46

Factor a perfect square on the left side:
(x + -5)(x + -5) = 46

Calculate the square root of the right side: 6.782329983

Break this problem into two subproblems by setting 
(x + -5) equal to 6.782329983 and -6.782329983.

Subproblem 1

x + -5 = 6.782329983 Simplifying x + -5 = 6.782329983 Reorder the terms: -5 + x = 6.782329983 Solving -5 + x = 6.782329983 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '5' to each side of the equation. -5 + 5 + x = 6.782329983 + 5 Combine like terms: -5 + 5 = 0 0 + x = 6.782329983 + 5 x = 6.782329983 + 5 Combine like terms: 6.782329983 + 5 = 11.782329983 x = 11.782329983 Simplifying x = 11.782329983

Subproblem 2

x + -5 = -6.782329983 Simplifying x + -5 = -6.782329983 Reorder the terms: -5 + x = -6.782329983 Solving -5 + x = -6.782329983 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '5' to each side of the equation. -5 + 5 + x = -6.782329983 + 5 Combine like terms: -5 + 5 = 0 0 + x = -6.782329983 + 5 x = -6.782329983 + 5 Combine like terms: -6.782329983 + 5 = -1.782329983 x = -1.782329983 Simplifying x = -1.782329983

Solution

The solution to the problem is based on the solutions from the subproblems. x = {11.782329983, -1.782329983}

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