r^2-21r-20=0

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Solution for r^2-21r-20=0 equation:


Simplifying
r2 + -21r + -20 = 0

Reorder the terms:
-20 + -21r + r2 = 0

Solving
-20 + -21r + r2 = 0

Solving for variable 'r'.

Begin completing the square.

Move the constant term to the right:

Add '20' to each side of the equation.
-20 + -21r + 20 + r2 = 0 + 20

Reorder the terms:
-20 + 20 + -21r + r2 = 0 + 20

Combine like terms: -20 + 20 = 0
0 + -21r + r2 = 0 + 20
-21r + r2 = 0 + 20

Combine like terms: 0 + 20 = 20
-21r + r2 = 20

The r term is -21r.  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.
-21r + 110.25 + r2 = 20 + 110.25

Reorder the terms:
110.25 + -21r + r2 = 20 + 110.25

Combine like terms: 20 + 110.25 = 130.25
110.25 + -21r + r2 = 130.25

Factor a perfect square on the left side:
(r + -10.5)(r + -10.5) = 130.25

Calculate the square root of the right side: 11.412712211

Break this problem into two subproblems by setting 
(r + -10.5) equal to 11.412712211 and -11.412712211.

Subproblem 1

r + -10.5 = 11.412712211 Simplifying r + -10.5 = 11.412712211 Reorder the terms: -10.5 + r = 11.412712211 Solving -10.5 + r = 11.412712211 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '10.5' to each side of the equation. -10.5 + 10.5 + r = 11.412712211 + 10.5 Combine like terms: -10.5 + 10.5 = 0.0 0.0 + r = 11.412712211 + 10.5 r = 11.412712211 + 10.5 Combine like terms: 11.412712211 + 10.5 = 21.912712211 r = 21.912712211 Simplifying r = 21.912712211

Subproblem 2

r + -10.5 = -11.412712211 Simplifying r + -10.5 = -11.412712211 Reorder the terms: -10.5 + r = -11.412712211 Solving -10.5 + r = -11.412712211 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '10.5' to each side of the equation. -10.5 + 10.5 + r = -11.412712211 + 10.5 Combine like terms: -10.5 + 10.5 = 0.0 0.0 + r = -11.412712211 + 10.5 r = -11.412712211 + 10.5 Combine like terms: -11.412712211 + 10.5 = -0.912712211 r = -0.912712211 Simplifying r = -0.912712211

Solution

The solution to the problem is based on the solutions from the subproblems. r = {21.912712211, -0.912712211}

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