5r^2-4r-20=0

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Solution for 5r^2-4r-20=0 equation:


Simplifying
5r2 + -4r + -20 = 0

Reorder the terms:
-20 + -4r + 5r2 = 0

Solving
-20 + -4r + 5r2 = 0

Solving for variable 'r'.

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

Divide each side by '5'.
-4 + -0.8r + r2 = 0

Move the constant term to the right:

Add '4' to each side of the equation.
-4 + -0.8r + 4 + r2 = 0 + 4

Reorder the terms:
-4 + 4 + -0.8r + r2 = 0 + 4

Combine like terms: -4 + 4 = 0
0 + -0.8r + r2 = 0 + 4
-0.8r + r2 = 0 + 4

Combine like terms: 0 + 4 = 4
-0.8r + r2 = 4

The r term is -0.8r.  Take half its coefficient (-0.4).
Square it (0.16) and add it to both sides.

Add '0.16' to each side of the equation.
-0.8r + 0.16 + r2 = 4 + 0.16

Reorder the terms:
0.16 + -0.8r + r2 = 4 + 0.16

Combine like terms: 4 + 0.16 = 4.16
0.16 + -0.8r + r2 = 4.16

Factor a perfect square on the left side:
(r + -0.4)(r + -0.4) = 4.16

Calculate the square root of the right side: 2.039607805

Break this problem into two subproblems by setting 
(r + -0.4) equal to 2.039607805 and -2.039607805.

Subproblem 1

r + -0.4 = 2.039607805 Simplifying r + -0.4 = 2.039607805 Reorder the terms: -0.4 + r = 2.039607805 Solving -0.4 + r = 2.039607805 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '0.4' to each side of the equation. -0.4 + 0.4 + r = 2.039607805 + 0.4 Combine like terms: -0.4 + 0.4 = 0.0 0.0 + r = 2.039607805 + 0.4 r = 2.039607805 + 0.4 Combine like terms: 2.039607805 + 0.4 = 2.439607805 r = 2.439607805 Simplifying r = 2.439607805

Subproblem 2

r + -0.4 = -2.039607805 Simplifying r + -0.4 = -2.039607805 Reorder the terms: -0.4 + r = -2.039607805 Solving -0.4 + r = -2.039607805 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '0.4' to each side of the equation. -0.4 + 0.4 + r = -2.039607805 + 0.4 Combine like terms: -0.4 + 0.4 = 0.0 0.0 + r = -2.039607805 + 0.4 r = -2.039607805 + 0.4 Combine like terms: -2.039607805 + 0.4 = -1.639607805 r = -1.639607805 Simplifying r = -1.639607805

Solution

The solution to the problem is based on the solutions from the subproblems. r = {2.439607805, -1.639607805}

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