r^2+18r=21

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Solution for r^2+18r=21 equation:


Simplifying
r2 + 18r = 21

Reorder the terms:
18r + r2 = 21

Solving
18r + r2 = 21

Solving for variable 'r'.

Reorder the terms:
-21 + 18r + r2 = 21 + -21

Combine like terms: 21 + -21 = 0
-21 + 18r + r2 = 0

Begin completing the square.

Move the constant term to the right:

Add '21' to each side of the equation.
-21 + 18r + 21 + r2 = 0 + 21

Reorder the terms:
-21 + 21 + 18r + r2 = 0 + 21

Combine like terms: -21 + 21 = 0
0 + 18r + r2 = 0 + 21
18r + r2 = 0 + 21

Combine like terms: 0 + 21 = 21
18r + r2 = 21

The r term is 18r.  Take half its coefficient (9).
Square it (81) and add it to both sides.

Add '81' to each side of the equation.
18r + 81 + r2 = 21 + 81

Reorder the terms:
81 + 18r + r2 = 21 + 81

Combine like terms: 21 + 81 = 102
81 + 18r + r2 = 102

Factor a perfect square on the left side:
(r + 9)(r + 9) = 102

Calculate the square root of the right side: 10.099504938

Break this problem into two subproblems by setting 
(r + 9) equal to 10.099504938 and -10.099504938.

Subproblem 1

r + 9 = 10.099504938 Simplifying r + 9 = 10.099504938 Reorder the terms: 9 + r = 10.099504938 Solving 9 + r = 10.099504938 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '-9' to each side of the equation. 9 + -9 + r = 10.099504938 + -9 Combine like terms: 9 + -9 = 0 0 + r = 10.099504938 + -9 r = 10.099504938 + -9 Combine like terms: 10.099504938 + -9 = 1.099504938 r = 1.099504938 Simplifying r = 1.099504938

Subproblem 2

r + 9 = -10.099504938 Simplifying r + 9 = -10.099504938 Reorder the terms: 9 + r = -10.099504938 Solving 9 + r = -10.099504938 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '-9' to each side of the equation. 9 + -9 + r = -10.099504938 + -9 Combine like terms: 9 + -9 = 0 0 + r = -10.099504938 + -9 r = -10.099504938 + -9 Combine like terms: -10.099504938 + -9 = -19.099504938 r = -19.099504938 Simplifying r = -19.099504938

Solution

The solution to the problem is based on the solutions from the subproblems. r = {1.099504938, -19.099504938}

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