13r^2-5r-21=0

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Solution for 13r^2-5r-21=0 equation:


Simplifying
13r2 + -5r + -21 = 0

Reorder the terms:
-21 + -5r + 13r2 = 0

Solving
-21 + -5r + 13r2 = 0

Solving for variable 'r'.

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

Divide each side by '13'.
-1.615384615 + -0.3846153846r + r2 = 0

Move the constant term to the right:

Add '1.615384615' to each side of the equation.
-1.615384615 + -0.3846153846r + 1.615384615 + r2 = 0 + 1.615384615

Reorder the terms:
-1.615384615 + 1.615384615 + -0.3846153846r + r2 = 0 + 1.615384615

Combine like terms: -1.615384615 + 1.615384615 = 0.000000000
0.000000000 + -0.3846153846r + r2 = 0 + 1.615384615
-0.3846153846r + r2 = 0 + 1.615384615

Combine like terms: 0 + 1.615384615 = 1.615384615
-0.3846153846r + r2 = 1.615384615

The r term is -0.3846153846r.  Take half its coefficient (-0.1923076923).
Square it (0.03698224852) and add it to both sides.

Add '0.03698224852' to each side of the equation.
-0.3846153846r + 0.03698224852 + r2 = 1.615384615 + 0.03698224852

Reorder the terms:
0.03698224852 + -0.3846153846r + r2 = 1.615384615 + 0.03698224852

Combine like terms: 1.615384615 + 0.03698224852 = 1.65236686352
0.03698224852 + -0.3846153846r + r2 = 1.65236686352

Factor a perfect square on the left side:
(r + -0.1923076923)(r + -0.1923076923) = 1.65236686352

Calculate the square root of the right side: 1.285444228

Break this problem into two subproblems by setting 
(r + -0.1923076923) equal to 1.285444228 and -1.285444228.

Subproblem 1

r + -0.1923076923 = 1.285444228 Simplifying r + -0.1923076923 = 1.285444228 Reorder the terms: -0.1923076923 + r = 1.285444228 Solving -0.1923076923 + r = 1.285444228 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '0.1923076923' to each side of the equation. -0.1923076923 + 0.1923076923 + r = 1.285444228 + 0.1923076923 Combine like terms: -0.1923076923 + 0.1923076923 = 0.0000000000 0.0000000000 + r = 1.285444228 + 0.1923076923 r = 1.285444228 + 0.1923076923 Combine like terms: 1.285444228 + 0.1923076923 = 1.4777519203 r = 1.4777519203 Simplifying r = 1.4777519203

Subproblem 2

r + -0.1923076923 = -1.285444228 Simplifying r + -0.1923076923 = -1.285444228 Reorder the terms: -0.1923076923 + r = -1.285444228 Solving -0.1923076923 + r = -1.285444228 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '0.1923076923' to each side of the equation. -0.1923076923 + 0.1923076923 + r = -1.285444228 + 0.1923076923 Combine like terms: -0.1923076923 + 0.1923076923 = 0.0000000000 0.0000000000 + r = -1.285444228 + 0.1923076923 r = -1.285444228 + 0.1923076923 Combine like terms: -1.285444228 + 0.1923076923 = -1.0931365357 r = -1.0931365357 Simplifying r = -1.0931365357

Solution

The solution to the problem is based on the solutions from the subproblems. r = {1.4777519203, -1.0931365357}

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