420=7.2x-0.03x^2

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Solution for 420=7.2x-0.03x^2 equation:


Simplifying
420 = 7.2x + -0.03x2

Solving
420 = 7.2x + -0.03x2

Solving for variable 'x'.

Reorder the terms:
420 + -7.2x + 0.03x2 = 7.2x + -7.2x + -0.03x2 + 0.03x2

Combine like terms: 7.2x + -7.2x = 0.0
420 + -7.2x + 0.03x2 = 0.0 + -0.03x2 + 0.03x2
420 + -7.2x + 0.03x2 = -0.03x2 + 0.03x2

Combine like terms: -0.03x2 + 0.03x2 = 0.00
420 + -7.2x + 0.03x2 = 0.00

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

Divide each side by '0.03'.
14000 + -240x + x2 = 0

Move the constant term to the right:

Add '-14000' to each side of the equation.
14000 + -240x + -14000 + x2 = 0 + -14000

Reorder the terms:
14000 + -14000 + -240x + x2 = 0 + -14000

Combine like terms: 14000 + -14000 = 0
0 + -240x + x2 = 0 + -14000
-240x + x2 = 0 + -14000

Combine like terms: 0 + -14000 = -14000
-240x + x2 = -14000

The x term is -240x.  Take half its coefficient (-120).
Square it (14400) and add it to both sides.

Add '14400' to each side of the equation.
-240x + 14400 + x2 = -14000 + 14400

Reorder the terms:
14400 + -240x + x2 = -14000 + 14400

Combine like terms: -14000 + 14400 = 400
14400 + -240x + x2 = 400

Factor a perfect square on the left side:
(x + -120)(x + -120) = 400

Calculate the square root of the right side: 20

Break this problem into two subproblems by setting 
(x + -120) equal to 20 and -20.

Subproblem 1

x + -120 = 20 Simplifying x + -120 = 20 Reorder the terms: -120 + x = 20 Solving -120 + x = 20 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '120' to each side of the equation. -120 + 120 + x = 20 + 120 Combine like terms: -120 + 120 = 0 0 + x = 20 + 120 x = 20 + 120 Combine like terms: 20 + 120 = 140 x = 140 Simplifying x = 140

Subproblem 2

x + -120 = -20 Simplifying x + -120 = -20 Reorder the terms: -120 + x = -20 Solving -120 + x = -20 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '120' to each side of the equation. -120 + 120 + x = -20 + 120 Combine like terms: -120 + 120 = 0 0 + x = -20 + 120 x = -20 + 120 Combine like terms: -20 + 120 = 100 x = 100 Simplifying x = 100

Solution

The solution to the problem is based on the solutions from the subproblems. x = {140, 100}

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