r^2-5r-8=0

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Solution for r^2-5r-8=0 equation: 


Simplifying
r2 + -5r + -8 = 0

Reorder the terms:
-8 + -5r + r2 = 0

Solving
-8 + -5r + r2 = 0

Solving for variable 'r'.

Begin completing the square.

Move the constant term to the right:

Add '8' to each side of the equation.
-8 + -5r + 8 + r2 = 0 + 8

Reorder the terms:
-8 + 8 + -5r + r2 = 0 + 8

Combine like terms: -8 + 8 = 0
0 + -5r + r2 = 0 + 8
-5r + r2 = 0 + 8

Combine like terms: 0 + 8 = 8
-5r + r2 = 8

The r term is -5r.  Take half its coefficient (-2.5).
Square it (6.25) and add it to both sides.

Add '6.25' to each side of the equation.
-5r + 6.25 + r2 = 8 + 6.25

Reorder the terms:
6.25 + -5r + r2 = 8 + 6.25

Combine like terms: 8 + 6.25 = 14.25
6.25 + -5r + r2 = 14.25

Factor a perfect square on the left side:
(r + -2.5)(r + -2.5) = 14.25

Calculate the square root of the right side: 3.774917218

Break this problem into two subproblems by setting 
(r + -2.5) equal to 3.774917218 and -3.774917218.


Subproblem 1
r + -2.5 = 3.774917218

Simplifying
r + -2.5 = 3.774917218

Reorder the terms:
-2.5 + r = 3.774917218

Solving
-2.5 + r = 3.774917218

Solving for variable 'r'.

Move all terms containing r to the left, all other terms to the right.

Add '2.5' to each side of the equation.
-2.5 + 2.5 + r = 3.774917218 + 2.5

Combine like terms: -2.5 + 2.5 = 0.0
0.0 + r = 3.774917218 + 2.5
r = 3.774917218 + 2.5

Combine like terms: 3.774917218 + 2.5 = 6.274917218
r = 6.274917218

Simplifying
r = 6.274917218


Subproblem 2
r + -2.5 = -3.774917218

Simplifying
r + -2.5 = -3.774917218

Reorder the terms:
-2.5 + r = -3.774917218

Solving
-2.5 + r = -3.774917218

Solving for variable 'r'.

Move all terms containing r to the left, all other terms to the right.

Add '2.5' to each side of the equation.
-2.5 + 2.5 + r = -3.774917218 + 2.5

Combine like terms: -2.5 + 2.5 = 0.0
0.0 + r = -3.774917218 + 2.5
r = -3.774917218 + 2.5

Combine like terms: -3.774917218 + 2.5 = -1.274917218
r = -1.274917218

Simplifying
r = -1.274917218


Solution
The solution to the problem is based on the solutions
from the subproblems.
r = {6.274917218, -1.274917218}

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