2r^2-13r-20=0

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Solution for 2r^2-13r-20=0 equation:


Simplifying
2r2 + -13r + -20 = 0

Reorder the terms:
-20 + -13r + 2r2 = 0

Solving
-20 + -13r + 2r2 = 0

Solving for variable 'r'.

Begin completing the square.  Divide all terms by
2 the coefficient of the squared term: 

Divide each side by '2'.
-10 + -6.5r + r2 = 0

Move the constant term to the right:

Add '10' to each side of the equation.
-10 + -6.5r + 10 + r2 = 0 + 10

Reorder the terms:
-10 + 10 + -6.5r + r2 = 0 + 10

Combine like terms: -10 + 10 = 0
0 + -6.5r + r2 = 0 + 10
-6.5r + r2 = 0 + 10

Combine like terms: 0 + 10 = 10
-6.5r + r2 = 10

The r term is -6.5r.  Take half its coefficient (-3.25).
Square it (10.5625) and add it to both sides.

Add '10.5625' to each side of the equation.
-6.5r + 10.5625 + r2 = 10 + 10.5625

Reorder the terms:
10.5625 + -6.5r + r2 = 10 + 10.5625

Combine like terms: 10 + 10.5625 = 20.5625
10.5625 + -6.5r + r2 = 20.5625

Factor a perfect square on the left side:
(r + -3.25)(r + -3.25) = 20.5625

Calculate the square root of the right side: 4.534589287

Break this problem into two subproblems by setting 
(r + -3.25) equal to 4.534589287 and -4.534589287.

Subproblem 1

r + -3.25 = 4.534589287 Simplifying r + -3.25 = 4.534589287 Reorder the terms: -3.25 + r = 4.534589287 Solving -3.25 + r = 4.534589287 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '3.25' to each side of the equation. -3.25 + 3.25 + r = 4.534589287 + 3.25 Combine like terms: -3.25 + 3.25 = 0.00 0.00 + r = 4.534589287 + 3.25 r = 4.534589287 + 3.25 Combine like terms: 4.534589287 + 3.25 = 7.784589287 r = 7.784589287 Simplifying r = 7.784589287

Subproblem 2

r + -3.25 = -4.534589287 Simplifying r + -3.25 = -4.534589287 Reorder the terms: -3.25 + r = -4.534589287 Solving -3.25 + r = -4.534589287 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '3.25' to each side of the equation. -3.25 + 3.25 + r = -4.534589287 + 3.25 Combine like terms: -3.25 + 3.25 = 0.00 0.00 + r = -4.534589287 + 3.25 r = -4.534589287 + 3.25 Combine like terms: -4.534589287 + 3.25 = -1.284589287 r = -1.284589287 Simplifying r = -1.284589287

Solution

The solution to the problem is based on the solutions from the subproblems. r = {7.784589287, -1.284589287}

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