-0.4x^2-4x+800=0

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Solution for -0.4x^2-4x+800=0 equation:


Simplifying
-0.4x2 + -4x + 800 = 0

Reorder the terms:
800 + -4x + -0.4x2 = 0

Solving
800 + -4x + -0.4x2 = 0

Solving for variable 'x'.

Begin completing the square.  Divide all terms by
-0.4 the coefficient of the squared term: 

Divide each side by '-0.4'.
-2000 + 10x + x2 = 0

Move the constant term to the right:

Add '2000' to each side of the equation.
-2000 + 10x + 2000 + x2 = 0 + 2000

Reorder the terms:
-2000 + 2000 + 10x + x2 = 0 + 2000

Combine like terms: -2000 + 2000 = 0
0 + 10x + x2 = 0 + 2000
10x + x2 = 0 + 2000

Combine like terms: 0 + 2000 = 2000
10x + x2 = 2000

The x term is 10x.  Take half its coefficient (5).
Square it (25) and add it to both sides.

Add '25' to each side of the equation.
10x + 25 + x2 = 2000 + 25

Reorder the terms:
25 + 10x + x2 = 2000 + 25

Combine like terms: 2000 + 25 = 2025
25 + 10x + x2 = 2025

Factor a perfect square on the left side:
(x + 5)(x + 5) = 2025

Calculate the square root of the right side: 45

Break this problem into two subproblems by setting 
(x + 5) equal to 45 and -45.

Subproblem 1

x + 5 = 45 Simplifying x + 5 = 45 Reorder the terms: 5 + x = 45 Solving 5 + x = 45 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-5' to each side of the equation. 5 + -5 + x = 45 + -5 Combine like terms: 5 + -5 = 0 0 + x = 45 + -5 x = 45 + -5 Combine like terms: 45 + -5 = 40 x = 40 Simplifying x = 40

Subproblem 2

x + 5 = -45 Simplifying x + 5 = -45 Reorder the terms: 5 + x = -45 Solving 5 + x = -45 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-5' to each side of the equation. 5 + -5 + x = -45 + -5 Combine like terms: 5 + -5 = 0 0 + x = -45 + -5 x = -45 + -5 Combine like terms: -45 + -5 = -50 x = -50 Simplifying x = -50

Solution

The solution to the problem is based on the solutions from the subproblems. x = {40, -50}

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