r^2-2r=90

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Solution for r^2-2r=90 equation:


Simplifying
r2 + -2r = 90

Reorder the terms:
-2r + r2 = 90

Solving
-2r + r2 = 90

Solving for variable 'r'.

Reorder the terms:
-90 + -2r + r2 = 90 + -90

Combine like terms: 90 + -90 = 0
-90 + -2r + r2 = 0

Begin completing the square.

Move the constant term to the right:

Add '90' to each side of the equation.
-90 + -2r + 90 + r2 = 0 + 90

Reorder the terms:
-90 + 90 + -2r + r2 = 0 + 90

Combine like terms: -90 + 90 = 0
0 + -2r + r2 = 0 + 90
-2r + r2 = 0 + 90

Combine like terms: 0 + 90 = 90
-2r + r2 = 90

The r term is -2r.  Take half its coefficient (-1).
Square it (1) and add it to both sides.

Add '1' to each side of the equation.
-2r + 1 + r2 = 90 + 1

Reorder the terms:
1 + -2r + r2 = 90 + 1

Combine like terms: 90 + 1 = 91
1 + -2r + r2 = 91

Factor a perfect square on the left side:
(r + -1)(r + -1) = 91

Calculate the square root of the right side: 9.539392014

Break this problem into two subproblems by setting 
(r + -1) equal to 9.539392014 and -9.539392014.

Subproblem 1

r + -1 = 9.539392014 Simplifying r + -1 = 9.539392014 Reorder the terms: -1 + r = 9.539392014 Solving -1 + r = 9.539392014 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '1' to each side of the equation. -1 + 1 + r = 9.539392014 + 1 Combine like terms: -1 + 1 = 0 0 + r = 9.539392014 + 1 r = 9.539392014 + 1 Combine like terms: 9.539392014 + 1 = 10.539392014 r = 10.539392014 Simplifying r = 10.539392014

Subproblem 2

r + -1 = -9.539392014 Simplifying r + -1 = -9.539392014 Reorder the terms: -1 + r = -9.539392014 Solving -1 + r = -9.539392014 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '1' to each side of the equation. -1 + 1 + r = -9.539392014 + 1 Combine like terms: -1 + 1 = 0 0 + r = -9.539392014 + 1 r = -9.539392014 + 1 Combine like terms: -9.539392014 + 1 = -8.539392014 r = -8.539392014 Simplifying r = -8.539392014

Solution

The solution to the problem is based on the solutions from the subproblems. r = {10.539392014, -8.539392014}

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