720=0.8x^2+4x

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Solution for 720=0.8x^2+4x equation:


Simplifying
720 = 0.8x2 + 4x

Reorder the terms:
720 = 4x + 0.8x2

Solving
720 = 4x + 0.8x2

Solving for variable 'x'.

Reorder the terms:
720 + -4x + -0.8x2 = 4x + -4x + 0.8x2 + -0.8x2

Combine like terms: 4x + -4x = 0
720 + -4x + -0.8x2 = 0 + 0.8x2 + -0.8x2
720 + -4x + -0.8x2 = 0.8x2 + -0.8x2

Combine like terms: 0.8x2 + -0.8x2 = 0.0
720 + -4x + -0.8x2 = 0.0

Begin completing the square.  Divide all terms by
-0.8 the coefficient of the squared term: 

Divide each side by '-0.8'.
-900 + 5x + x2 = 0

Move the constant term to the right:

Add '900' to each side of the equation.
-900 + 5x + 900 + x2 = 0 + 900

Reorder the terms:
-900 + 900 + 5x + x2 = 0 + 900

Combine like terms: -900 + 900 = 0
0 + 5x + x2 = 0 + 900
5x + x2 = 0 + 900

Combine like terms: 0 + 900 = 900
5x + x2 = 900

The x term is 5x.  Take half its coefficient (2.5).
Square it (6.25) and add it to both sides.

Add '6.25' to each side of the equation.
5x + 6.25 + x2 = 900 + 6.25

Reorder the terms:
6.25 + 5x + x2 = 900 + 6.25

Combine like terms: 900 + 6.25 = 906.25
6.25 + 5x + x2 = 906.25

Factor a perfect square on the left side:
(x + 2.5)(x + 2.5) = 906.25

Calculate the square root of the right side: 30.103986447

Break this problem into two subproblems by setting 
(x + 2.5) equal to 30.103986447 and -30.103986447.

Subproblem 1

x + 2.5 = 30.103986447 Simplifying x + 2.5 = 30.103986447 Reorder the terms: 2.5 + x = 30.103986447 Solving 2.5 + x = 30.103986447 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-2.5' to each side of the equation. 2.5 + -2.5 + x = 30.103986447 + -2.5 Combine like terms: 2.5 + -2.5 = 0.0 0.0 + x = 30.103986447 + -2.5 x = 30.103986447 + -2.5 Combine like terms: 30.103986447 + -2.5 = 27.603986447 x = 27.603986447 Simplifying x = 27.603986447

Subproblem 2

x + 2.5 = -30.103986447 Simplifying x + 2.5 = -30.103986447 Reorder the terms: 2.5 + x = -30.103986447 Solving 2.5 + x = -30.103986447 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-2.5' to each side of the equation. 2.5 + -2.5 + x = -30.103986447 + -2.5 Combine like terms: 2.5 + -2.5 = 0.0 0.0 + x = -30.103986447 + -2.5 x = -30.103986447 + -2.5 Combine like terms: -30.103986447 + -2.5 = -32.603986447 x = -32.603986447 Simplifying x = -32.603986447

Solution

The solution to the problem is based on the solutions from the subproblems. x = {27.603986447, -32.603986447}

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