(r-4)(r+8)=39

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Solution for (r-4)(r+8)=39 equation:


Simplifying
(r + -4)(r + 8) = 39

Reorder the terms:
(-4 + r)(r + 8) = 39

Reorder the terms:
(-4 + r)(8 + r) = 39

Multiply (-4 + r) * (8 + r)
(-4(8 + r) + r(8 + r)) = 39
((8 * -4 + r * -4) + r(8 + r)) = 39
((-32 + -4r) + r(8 + r)) = 39
(-32 + -4r + (8 * r + r * r)) = 39
(-32 + -4r + (8r + r2)) = 39

Combine like terms: -4r + 8r = 4r
(-32 + 4r + r2) = 39

Solving
-32 + 4r + r2 = 39

Solving for variable 'r'.

Reorder the terms:
-32 + -39 + 4r + r2 = 39 + -39

Combine like terms: -32 + -39 = -71
-71 + 4r + r2 = 39 + -39

Combine like terms: 39 + -39 = 0
-71 + 4r + r2 = 0

Begin completing the square.

Move the constant term to the right:

Add '71' to each side of the equation.
-71 + 4r + 71 + r2 = 0 + 71

Reorder the terms:
-71 + 71 + 4r + r2 = 0 + 71

Combine like terms: -71 + 71 = 0
0 + 4r + r2 = 0 + 71
4r + r2 = 0 + 71

Combine like terms: 0 + 71 = 71
4r + r2 = 71

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 = 71 + 4

Reorder the terms:
4 + 4r + r2 = 71 + 4

Combine like terms: 71 + 4 = 75
4 + 4r + r2 = 75

Factor a perfect square on the left side:
(r + 2)(r + 2) = 75

Calculate the square root of the right side: 8.660254038

Break this problem into two subproblems by setting 
(r + 2) equal to 8.660254038 and -8.660254038.

Subproblem 1

r + 2 = 8.660254038 Simplifying r + 2 = 8.660254038 Reorder the terms: 2 + r = 8.660254038 Solving 2 + r = 8.660254038 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 = 8.660254038 + -2 Combine like terms: 2 + -2 = 0 0 + r = 8.660254038 + -2 r = 8.660254038 + -2 Combine like terms: 8.660254038 + -2 = 6.660254038 r = 6.660254038 Simplifying r = 6.660254038

Subproblem 2

r + 2 = -8.660254038 Simplifying r + 2 = -8.660254038 Reorder the terms: 2 + r = -8.660254038 Solving 2 + r = -8.660254038 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 = -8.660254038 + -2 Combine like terms: 2 + -2 = 0 0 + r = -8.660254038 + -2 r = -8.660254038 + -2 Combine like terms: -8.660254038 + -2 = -10.660254038 r = -10.660254038 Simplifying r = -10.660254038

Solution

The solution to the problem is based on the solutions from the subproblems. r = {6.660254038, -10.660254038}

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