X^2-21X-90=0

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Solution for X^2-21X-90=0 equation:


Simplifying
X2 + -21X + -90 = 0

Reorder the terms:
-90 + -21X + X2 = 0

Solving
-90 + -21X + X2 = 0

Solving for variable 'X'.

Begin completing the square.

Move the constant term to the right:

Add '90' to each side of the equation.
-90 + -21X + 90 + X2 = 0 + 90

Reorder the terms:
-90 + 90 + -21X + X2 = 0 + 90

Combine like terms: -90 + 90 = 0
0 + -21X + X2 = 0 + 90
-21X + X2 = 0 + 90

Combine like terms: 0 + 90 = 90
-21X + X2 = 90

The X term is -21X.  Take half its coefficient (-10.5).
Square it (110.25) and add it to both sides.

Add '110.25' to each side of the equation.
-21X + 110.25 + X2 = 90 + 110.25

Reorder the terms:
110.25 + -21X + X2 = 90 + 110.25

Combine like terms: 90 + 110.25 = 200.25
110.25 + -21X + X2 = 200.25

Factor a perfect square on the left side:
(X + -10.5)(X + -10.5) = 200.25

Calculate the square root of the right side: 14.150971698

Break this problem into two subproblems by setting 
(X + -10.5) equal to 14.150971698 and -14.150971698.

Subproblem 1

X + -10.5 = 14.150971698 Simplifying X + -10.5 = 14.150971698 Reorder the terms: -10.5 + X = 14.150971698 Solving -10.5 + X = 14.150971698 Solving for variable 'X'. Move all terms containing X to the left, all other terms to the right. Add '10.5' to each side of the equation. -10.5 + 10.5 + X = 14.150971698 + 10.5 Combine like terms: -10.5 + 10.5 = 0.0 0.0 + X = 14.150971698 + 10.5 X = 14.150971698 + 10.5 Combine like terms: 14.150971698 + 10.5 = 24.650971698 X = 24.650971698 Simplifying X = 24.650971698

Subproblem 2

X + -10.5 = -14.150971698 Simplifying X + -10.5 = -14.150971698 Reorder the terms: -10.5 + X = -14.150971698 Solving -10.5 + X = -14.150971698 Solving for variable 'X'. Move all terms containing X to the left, all other terms to the right. Add '10.5' to each side of the equation. -10.5 + 10.5 + X = -14.150971698 + 10.5 Combine like terms: -10.5 + 10.5 = 0.0 0.0 + X = -14.150971698 + 10.5 X = -14.150971698 + 10.5 Combine like terms: -14.150971698 + 10.5 = -3.650971698 X = -3.650971698 Simplifying X = -3.650971698

Solution

The solution to the problem is based on the solutions from the subproblems. X = {24.650971698, -3.650971698}

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