400=0.1x^2+3x

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Solution for 400=0.1x^2+3x equation:


Simplifying
400 = 0.1x2 + 3x

Reorder the terms:
400 = 3x + 0.1x2

Solving
400 = 3x + 0.1x2

Solving for variable 'x'.

Reorder the terms:
400 + -3x + -0.1x2 = 3x + -3x + 0.1x2 + -0.1x2

Combine like terms: 3x + -3x = 0
400 + -3x + -0.1x2 = 0 + 0.1x2 + -0.1x2
400 + -3x + -0.1x2 = 0.1x2 + -0.1x2

Combine like terms: 0.1x2 + -0.1x2 = 0.0
400 + -3x + -0.1x2 = 0.0

Begin completing the square.  Divide all terms by
-0.1 the coefficient of the squared term: 

Divide each side by '-0.1'.
-4000 + 30x + x2 = 0

Move the constant term to the right:

Add '4000' to each side of the equation.
-4000 + 30x + 4000 + x2 = 0 + 4000

Reorder the terms:
-4000 + 4000 + 30x + x2 = 0 + 4000

Combine like terms: -4000 + 4000 = 0
0 + 30x + x2 = 0 + 4000
30x + x2 = 0 + 4000

Combine like terms: 0 + 4000 = 4000
30x + x2 = 4000

The x term is 30x.  Take half its coefficient (15).
Square it (225) and add it to both sides.

Add '225' to each side of the equation.
30x + 225 + x2 = 4000 + 225

Reorder the terms:
225 + 30x + x2 = 4000 + 225

Combine like terms: 4000 + 225 = 4225
225 + 30x + x2 = 4225

Factor a perfect square on the left side:
(x + 15)(x + 15) = 4225

Calculate the square root of the right side: 65

Break this problem into two subproblems by setting 
(x + 15) equal to 65 and -65.

Subproblem 1

x + 15 = 65 Simplifying x + 15 = 65 Reorder the terms: 15 + x = 65 Solving 15 + x = 65 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-15' to each side of the equation. 15 + -15 + x = 65 + -15 Combine like terms: 15 + -15 = 0 0 + x = 65 + -15 x = 65 + -15 Combine like terms: 65 + -15 = 50 x = 50 Simplifying x = 50

Subproblem 2

x + 15 = -65 Simplifying x + 15 = -65 Reorder the terms: 15 + x = -65 Solving 15 + x = -65 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-15' to each side of the equation. 15 + -15 + x = -65 + -15 Combine like terms: 15 + -15 = 0 0 + x = -65 + -15 x = -65 + -15 Combine like terms: -65 + -15 = -80 x = -80 Simplifying x = -80

Solution

The solution to the problem is based on the solutions from the subproblems. x = {50, -80}

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