(r-3)(r+7)=100

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Solution for (r-3)(r+7)=100 equation:


Simplifying
(r + -3)(r + 7) = 100

Reorder the terms:
(-3 + r)(r + 7) = 100

Reorder the terms:
(-3 + r)(7 + r) = 100

Multiply (-3 + r) * (7 + r)
(-3(7 + r) + r(7 + r)) = 100
((7 * -3 + r * -3) + r(7 + r)) = 100
((-21 + -3r) + r(7 + r)) = 100
(-21 + -3r + (7 * r + r * r)) = 100
(-21 + -3r + (7r + r2)) = 100

Combine like terms: -3r + 7r = 4r
(-21 + 4r + r2) = 100

Solving
-21 + 4r + r2 = 100

Solving for variable 'r'.

Reorder the terms:
-21 + -100 + 4r + r2 = 100 + -100

Combine like terms: -21 + -100 = -121
-121 + 4r + r2 = 100 + -100

Combine like terms: 100 + -100 = 0
-121 + 4r + r2 = 0

Begin completing the square.

Move the constant term to the right:

Add '121' to each side of the equation.
-121 + 4r + 121 + r2 = 0 + 121

Reorder the terms:
-121 + 121 + 4r + r2 = 0 + 121

Combine like terms: -121 + 121 = 0
0 + 4r + r2 = 0 + 121
4r + r2 = 0 + 121

Combine like terms: 0 + 121 = 121
4r + r2 = 121

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 = 121 + 4

Reorder the terms:
4 + 4r + r2 = 121 + 4

Combine like terms: 121 + 4 = 125
4 + 4r + r2 = 125

Factor a perfect square on the left side:
(r + 2)(r + 2) = 125

Calculate the square root of the right side: 11.180339887

Break this problem into two subproblems by setting 
(r + 2) equal to 11.180339887 and -11.180339887.

Subproblem 1

r + 2 = 11.180339887 Simplifying r + 2 = 11.180339887 Reorder the terms: 2 + r = 11.180339887 Solving 2 + r = 11.180339887 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 = 11.180339887 + -2 Combine like terms: 2 + -2 = 0 0 + r = 11.180339887 + -2 r = 11.180339887 + -2 Combine like terms: 11.180339887 + -2 = 9.180339887 r = 9.180339887 Simplifying r = 9.180339887

Subproblem 2

r + 2 = -11.180339887 Simplifying r + 2 = -11.180339887 Reorder the terms: 2 + r = -11.180339887 Solving 2 + r = -11.180339887 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 = -11.180339887 + -2 Combine like terms: 2 + -2 = 0 0 + r = -11.180339887 + -2 r = -11.180339887 + -2 Combine like terms: -11.180339887 + -2 = -13.180339887 r = -13.180339887 Simplifying r = -13.180339887

Solution

The solution to the problem is based on the solutions from the subproblems. r = {9.180339887, -13.180339887}

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