7b^2=21+9b

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Solution for 7b^2=21+9b equation:


Simplifying
7b2 = 21 + 9b

Solving
7b2 = 21 + 9b

Solving for variable 'b'.

Reorder the terms:
-21 + -9b + 7b2 = 21 + 9b + -21 + -9b

Reorder the terms:
-21 + -9b + 7b2 = 21 + -21 + 9b + -9b

Combine like terms: 21 + -21 = 0
-21 + -9b + 7b2 = 0 + 9b + -9b
-21 + -9b + 7b2 = 9b + -9b

Combine like terms: 9b + -9b = 0
-21 + -9b + 7b2 = 0

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

Divide each side by '7'.
-3 + -1.285714286b + b2 = 0

Move the constant term to the right:

Add '3' to each side of the equation.
-3 + -1.285714286b + 3 + b2 = 0 + 3

Reorder the terms:
-3 + 3 + -1.285714286b + b2 = 0 + 3

Combine like terms: -3 + 3 = 0
0 + -1.285714286b + b2 = 0 + 3
-1.285714286b + b2 = 0 + 3

Combine like terms: 0 + 3 = 3
-1.285714286b + b2 = 3

The b term is -1.285714286b.  Take half its coefficient (-0.642857143).
Square it (0.4132653063) and add it to both sides.

Add '0.4132653063' to each side of the equation.
-1.285714286b + 0.4132653063 + b2 = 3 + 0.4132653063

Reorder the terms:
0.4132653063 + -1.285714286b + b2 = 3 + 0.4132653063

Combine like terms: 3 + 0.4132653063 = 3.4132653063
0.4132653063 + -1.285714286b + b2 = 3.4132653063

Factor a perfect square on the left side:
(b + -0.642857143)(b + -0.642857143) = 3.4132653063

Calculate the square root of the right side: 1.847502451

Break this problem into two subproblems by setting 
(b + -0.642857143) equal to 1.847502451 and -1.847502451.

Subproblem 1

b + -0.642857143 = 1.847502451 Simplifying b + -0.642857143 = 1.847502451 Reorder the terms: -0.642857143 + b = 1.847502451 Solving -0.642857143 + b = 1.847502451 Solving for variable 'b'. Move all terms containing b to the left, all other terms to the right. Add '0.642857143' to each side of the equation. -0.642857143 + 0.642857143 + b = 1.847502451 + 0.642857143 Combine like terms: -0.642857143 + 0.642857143 = 0.000000000 0.000000000 + b = 1.847502451 + 0.642857143 b = 1.847502451 + 0.642857143 Combine like terms: 1.847502451 + 0.642857143 = 2.490359594 b = 2.490359594 Simplifying b = 2.490359594

Subproblem 2

b + -0.642857143 = -1.847502451 Simplifying b + -0.642857143 = -1.847502451 Reorder the terms: -0.642857143 + b = -1.847502451 Solving -0.642857143 + b = -1.847502451 Solving for variable 'b'. Move all terms containing b to the left, all other terms to the right. Add '0.642857143' to each side of the equation. -0.642857143 + 0.642857143 + b = -1.847502451 + 0.642857143 Combine like terms: -0.642857143 + 0.642857143 = 0.000000000 0.000000000 + b = -1.847502451 + 0.642857143 b = -1.847502451 + 0.642857143 Combine like terms: -1.847502451 + 0.642857143 = -1.204645308 b = -1.204645308 Simplifying b = -1.204645308

Solution

The solution to the problem is based on the solutions from the subproblems. b = {2.490359594, -1.204645308}

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