81=b(b+18)

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Solution for 81=b(b+18) equation:


Simplifying
81 = b(b + 18)

Reorder the terms:
81 = b(18 + b)
81 = (18 * b + b * b)
81 = (18b + b2)

Solving
81 = 18b + b2

Solving for variable 'b'.

Reorder the terms:
81 + -18b + -1b2 = 18b + -18b + b2 + -1b2

Combine like terms: 18b + -18b = 0
81 + -18b + -1b2 = 0 + b2 + -1b2
81 + -18b + -1b2 = b2 + -1b2

Combine like terms: b2 + -1b2 = 0
81 + -18b + -1b2 = 0

Begin completing the square.  Divide all terms by
-1 the coefficient of the squared term: 

Divide each side by '-1'.
-81 + 18b + b2 = 0

Move the constant term to the right:

Add '81' to each side of the equation.
-81 + 18b + 81 + b2 = 0 + 81

Reorder the terms:
-81 + 81 + 18b + b2 = 0 + 81

Combine like terms: -81 + 81 = 0
0 + 18b + b2 = 0 + 81
18b + b2 = 0 + 81

Combine like terms: 0 + 81 = 81
18b + b2 = 81

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

Add '81' to each side of the equation.
18b + 81 + b2 = 81 + 81

Reorder the terms:
81 + 18b + b2 = 81 + 81

Combine like terms: 81 + 81 = 162
81 + 18b + b2 = 162

Factor a perfect square on the left side:
(b + 9)(b + 9) = 162

Calculate the square root of the right side: 12.727922061

Break this problem into two subproblems by setting 
(b + 9) equal to 12.727922061 and -12.727922061.

Subproblem 1

b + 9 = 12.727922061 Simplifying b + 9 = 12.727922061 Reorder the terms: 9 + b = 12.727922061 Solving 9 + b = 12.727922061 Solving for variable 'b'. Move all terms containing b to the left, all other terms to the right. Add '-9' to each side of the equation. 9 + -9 + b = 12.727922061 + -9 Combine like terms: 9 + -9 = 0 0 + b = 12.727922061 + -9 b = 12.727922061 + -9 Combine like terms: 12.727922061 + -9 = 3.727922061 b = 3.727922061 Simplifying b = 3.727922061

Subproblem 2

b + 9 = -12.727922061 Simplifying b + 9 = -12.727922061 Reorder the terms: 9 + b = -12.727922061 Solving 9 + b = -12.727922061 Solving for variable 'b'. Move all terms containing b to the left, all other terms to the right. Add '-9' to each side of the equation. 9 + -9 + b = -12.727922061 + -9 Combine like terms: 9 + -9 = 0 0 + b = -12.727922061 + -9 b = -12.727922061 + -9 Combine like terms: -12.727922061 + -9 = -21.727922061 b = -21.727922061 Simplifying b = -21.727922061

Solution

The solution to the problem is based on the solutions from the subproblems. b = {3.727922061, -21.727922061}

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