r^2-4r-15=0

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Solution for r^2-4r-15=0 equation:


Simplifying
r2 + -4r + -15 = 0

Reorder the terms:
-15 + -4r + r2 = 0

Solving
-15 + -4r + r2 = 0

Solving for variable 'r'.

Begin completing the square.

Move the constant term to the right:

Add '15' to each side of the equation.
-15 + -4r + 15 + r2 = 0 + 15

Reorder the terms:
-15 + 15 + -4r + r2 = 0 + 15

Combine like terms: -15 + 15 = 0
0 + -4r + r2 = 0 + 15
-4r + r2 = 0 + 15

Combine like terms: 0 + 15 = 15
-4r + r2 = 15

The r term is -4r.  Take half its coefficient (-2).
Square it (4) and add it to both sides.

Add '4' to each side of the equation.
-4r + 4 + r2 = 15 + 4

Reorder the terms:
4 + -4r + r2 = 15 + 4

Combine like terms: 15 + 4 = 19
4 + -4r + r2 = 19

Factor a perfect square on the left side:
(r + -2)(r + -2) = 19

Calculate the square root of the right side: 4.358898944

Break this problem into two subproblems by setting 
(r + -2) equal to 4.358898944 and -4.358898944.

Subproblem 1

r + -2 = 4.358898944 Simplifying r + -2 = 4.358898944 Reorder the terms: -2 + r = 4.358898944 Solving -2 + r = 4.358898944 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '2' to each side of the equation. -2 + 2 + r = 4.358898944 + 2 Combine like terms: -2 + 2 = 0 0 + r = 4.358898944 + 2 r = 4.358898944 + 2 Combine like terms: 4.358898944 + 2 = 6.358898944 r = 6.358898944 Simplifying r = 6.358898944

Subproblem 2

r + -2 = -4.358898944 Simplifying r + -2 = -4.358898944 Reorder the terms: -2 + r = -4.358898944 Solving -2 + r = -4.358898944 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '2' to each side of the equation. -2 + 2 + r = -4.358898944 + 2 Combine like terms: -2 + 2 = 0 0 + r = -4.358898944 + 2 r = -4.358898944 + 2 Combine like terms: -4.358898944 + 2 = -2.358898944 r = -2.358898944 Simplifying r = -2.358898944

Solution

The solution to the problem is based on the solutions from the subproblems. r = {6.358898944, -2.358898944}

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