-(x-2.1)(5x-7.5)=0

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Solution for -(x-2.1)(5x-7.5)=0 equation:


Simplifying
-1(x + -2.1)(5x + -7.5) = 0

Reorder the terms:
-1(-2.1 + x)(5x + -7.5) = 0

Reorder the terms:
-1(-2.1 + x)(-7.5 + 5x) = 0

Multiply (-2.1 + x) * (-7.5 + 5x)
-1(-2.1(-7.5 + 5x) + x(-7.5 + 5x)) = 0
-1((-7.5 * -2.1 + 5x * -2.1) + x(-7.5 + 5x)) = 0
-1((15.75 + -10.5x) + x(-7.5 + 5x)) = 0
-1(15.75 + -10.5x + (-7.5 * x + 5x * x)) = 0
-1(15.75 + -10.5x + (-7.5x + 5x2)) = 0

Combine like terms: -10.5x + -7.5x = -18x
-1(15.75 + -18x + 5x2) = 0
(15.75 * -1 + -18x * -1 + 5x2 * -1) = 0
(-15.75 + 18x + -5x2) = 0

Solving
-15.75 + 18x + -5x2 = 0

Solving for variable 'x'.

Begin completing the square.  Divide all terms by
-5 the coefficient of the squared term: 

Divide each side by '-5'.
3.15 + -3.6x + x2 = 0

Move the constant term to the right:

Add '-3.15' to each side of the equation.
3.15 + -3.6x + -3.15 + x2 = 0 + -3.15

Reorder the terms:
3.15 + -3.15 + -3.6x + x2 = 0 + -3.15

Combine like terms: 3.15 + -3.15 = 0.00
0.00 + -3.6x + x2 = 0 + -3.15
-3.6x + x2 = 0 + -3.15

Combine like terms: 0 + -3.15 = -3.15
-3.6x + x2 = -3.15

The x term is -3.6x.  Take half its coefficient (-1.8).
Square it (3.24) and add it to both sides.

Add '3.24' to each side of the equation.
-3.6x + 3.24 + x2 = -3.15 + 3.24

Reorder the terms:
3.24 + -3.6x + x2 = -3.15 + 3.24

Combine like terms: -3.15 + 3.24 = 0.09
3.24 + -3.6x + x2 = 0.09

Factor a perfect square on the left side:
(x + -1.8)(x + -1.8) = 0.09

Calculate the square root of the right side: 0.3

Break this problem into two subproblems by setting 
(x + -1.8) equal to 0.3 and -0.3.

Subproblem 1

x + -1.8 = 0.3 Simplifying x + -1.8 = 0.3 Reorder the terms: -1.8 + x = 0.3 Solving -1.8 + x = 0.3 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '1.8' to each side of the equation. -1.8 + 1.8 + x = 0.3 + 1.8 Combine like terms: -1.8 + 1.8 = 0.0 0.0 + x = 0.3 + 1.8 x = 0.3 + 1.8 Combine like terms: 0.3 + 1.8 = 2.1 x = 2.1 Simplifying x = 2.1

Subproblem 2

x + -1.8 = -0.3 Simplifying x + -1.8 = -0.3 Reorder the terms: -1.8 + x = -0.3 Solving -1.8 + x = -0.3 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '1.8' to each side of the equation. -1.8 + 1.8 + x = -0.3 + 1.8 Combine like terms: -1.8 + 1.8 = 0.0 0.0 + x = -0.3 + 1.8 x = -0.3 + 1.8 Combine like terms: -0.3 + 1.8 = 1.5 x = 1.5 Simplifying x = 1.5

Solution

The solution to the problem is based on the solutions from the subproblems. x = {2.1, 1.5}

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