r^2+8r-15=0

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Solution for r^2+8r-15=0 equation:


Simplifying
r2 + 8r + -15 = 0

Reorder the terms:
-15 + 8r + r2 = 0

Solving
-15 + 8r + 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 + 8r + 15 + r2 = 0 + 15

Reorder the terms:
-15 + 15 + 8r + r2 = 0 + 15

Combine like terms: -15 + 15 = 0
0 + 8r + r2 = 0 + 15
8r + r2 = 0 + 15

Combine like terms: 0 + 15 = 15
8r + r2 = 15

The r term is 8r.  Take half its coefficient (4).
Square it (16) and add it to both sides.

Add '16' to each side of the equation.
8r + 16 + r2 = 15 + 16

Reorder the terms:
16 + 8r + r2 = 15 + 16

Combine like terms: 15 + 16 = 31
16 + 8r + r2 = 31

Factor a perfect square on the left side:
(r + 4)(r + 4) = 31

Calculate the square root of the right side: 5.567764363

Break this problem into two subproblems by setting 
(r + 4) equal to 5.567764363 and -5.567764363.

Subproblem 1

r + 4 = 5.567764363 Simplifying r + 4 = 5.567764363 Reorder the terms: 4 + r = 5.567764363 Solving 4 + r = 5.567764363 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '-4' to each side of the equation. 4 + -4 + r = 5.567764363 + -4 Combine like terms: 4 + -4 = 0 0 + r = 5.567764363 + -4 r = 5.567764363 + -4 Combine like terms: 5.567764363 + -4 = 1.567764363 r = 1.567764363 Simplifying r = 1.567764363

Subproblem 2

r + 4 = -5.567764363 Simplifying r + 4 = -5.567764363 Reorder the terms: 4 + r = -5.567764363 Solving 4 + r = -5.567764363 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '-4' to each side of the equation. 4 + -4 + r = -5.567764363 + -4 Combine like terms: 4 + -4 = 0 0 + r = -5.567764363 + -4 r = -5.567764363 + -4 Combine like terms: -5.567764363 + -4 = -9.567764363 r = -9.567764363 Simplifying r = -9.567764363

Solution

The solution to the problem is based on the solutions from the subproblems. r = {1.567764363, -9.567764363}

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