90=12x-.2x^2

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Solution for 90=12x-.2x^2 equation:


Simplifying
90 = 12x + -0.2x2

Solving
90 = 12x + -0.2x2

Solving for variable 'x'.

Reorder the terms:
90 + -12x + 0.2x2 = 12x + -12x + -0.2x2 + 0.2x2

Combine like terms: 12x + -12x = 0
90 + -12x + 0.2x2 = 0 + -0.2x2 + 0.2x2
90 + -12x + 0.2x2 = -0.2x2 + 0.2x2

Combine like terms: -0.2x2 + 0.2x2 = 0.0
90 + -12x + 0.2x2 = 0.0

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

Divide each side by '0.2'.
450 + -60x + x2 = 0

Move the constant term to the right:

Add '-450' to each side of the equation.
450 + -60x + -450 + x2 = 0 + -450

Reorder the terms:
450 + -450 + -60x + x2 = 0 + -450

Combine like terms: 450 + -450 = 0
0 + -60x + x2 = 0 + -450
-60x + x2 = 0 + -450

Combine like terms: 0 + -450 = -450
-60x + x2 = -450

The x term is -60x.  Take half its coefficient (-30).
Square it (900) and add it to both sides.

Add '900' to each side of the equation.
-60x + 900 + x2 = -450 + 900

Reorder the terms:
900 + -60x + x2 = -450 + 900

Combine like terms: -450 + 900 = 450
900 + -60x + x2 = 450

Factor a perfect square on the left side:
(x + -30)(x + -30) = 450

Calculate the square root of the right side: 21.213203436

Break this problem into two subproblems by setting 
(x + -30) equal to 21.213203436 and -21.213203436.

Subproblem 1

x + -30 = 21.213203436 Simplifying x + -30 = 21.213203436 Reorder the terms: -30 + x = 21.213203436 Solving -30 + x = 21.213203436 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '30' to each side of the equation. -30 + 30 + x = 21.213203436 + 30 Combine like terms: -30 + 30 = 0 0 + x = 21.213203436 + 30 x = 21.213203436 + 30 Combine like terms: 21.213203436 + 30 = 51.213203436 x = 51.213203436 Simplifying x = 51.213203436

Subproblem 2

x + -30 = -21.213203436 Simplifying x + -30 = -21.213203436 Reorder the terms: -30 + x = -21.213203436 Solving -30 + x = -21.213203436 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '30' to each side of the equation. -30 + 30 + x = -21.213203436 + 30 Combine like terms: -30 + 30 = 0 0 + x = -21.213203436 + 30 x = -21.213203436 + 30 Combine like terms: -21.213203436 + 30 = 8.786796564 x = 8.786796564 Simplifying x = 8.786796564

Solution

The solution to the problem is based on the solutions from the subproblems. x = {51.213203436, 8.786796564}

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