20.8=21x-4.9x^2

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Solution for 20.8=21x-4.9x^2 equation:


Simplifying
20.8 = 21x + -4.9x2

Solving
20.8 = 21x + -4.9x2

Solving for variable 'x'.

Reorder the terms:
20.8 + -21x + 4.9x2 = 21x + -21x + -4.9x2 + 4.9x2

Combine like terms: 21x + -21x = 0
20.8 + -21x + 4.9x2 = 0 + -4.9x2 + 4.9x2
20.8 + -21x + 4.9x2 = -4.9x2 + 4.9x2

Combine like terms: -4.9x2 + 4.9x2 = 0.0
20.8 + -21x + 4.9x2 = 0.0

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

Divide each side by '4.9'.
4.244897959 + -4.285714286x + x2 = 0

Move the constant term to the right:

Add '-4.244897959' to each side of the equation.
4.244897959 + -4.285714286x + -4.244897959 + x2 = 0 + -4.244897959

Reorder the terms:
4.244897959 + -4.244897959 + -4.285714286x + x2 = 0 + -4.244897959

Combine like terms: 4.244897959 + -4.244897959 = 0.000000000
0.000000000 + -4.285714286x + x2 = 0 + -4.244897959
-4.285714286x + x2 = 0 + -4.244897959

Combine like terms: 0 + -4.244897959 = -4.244897959
-4.285714286x + x2 = -4.244897959

The x term is -4.285714286x.  Take half its coefficient (-2.142857143).
Square it (4.591836735) and add it to both sides.

Add '4.591836735' to each side of the equation.
-4.285714286x + 4.591836735 + x2 = -4.244897959 + 4.591836735

Reorder the terms:
4.591836735 + -4.285714286x + x2 = -4.244897959 + 4.591836735

Combine like terms: -4.244897959 + 4.591836735 = 0.346938776
4.591836735 + -4.285714286x + x2 = 0.346938776

Factor a perfect square on the left side:
(x + -2.142857143)(x + -2.142857143) = 0.346938776

Calculate the square root of the right side: 0.58901509

Break this problem into two subproblems by setting 
(x + -2.142857143) equal to 0.58901509 and -0.58901509.

Subproblem 1

x + -2.142857143 = 0.58901509 Simplifying x + -2.142857143 = 0.58901509 Reorder the terms: -2.142857143 + x = 0.58901509 Solving -2.142857143 + x = 0.58901509 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '2.142857143' to each side of the equation. -2.142857143 + 2.142857143 + x = 0.58901509 + 2.142857143 Combine like terms: -2.142857143 + 2.142857143 = 0.000000000 0.000000000 + x = 0.58901509 + 2.142857143 x = 0.58901509 + 2.142857143 Combine like terms: 0.58901509 + 2.142857143 = 2.731872233 x = 2.731872233 Simplifying x = 2.731872233

Subproblem 2

x + -2.142857143 = -0.58901509 Simplifying x + -2.142857143 = -0.58901509 Reorder the terms: -2.142857143 + x = -0.58901509 Solving -2.142857143 + x = -0.58901509 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '2.142857143' to each side of the equation. -2.142857143 + 2.142857143 + x = -0.58901509 + 2.142857143 Combine like terms: -2.142857143 + 2.142857143 = 0.000000000 0.000000000 + x = -0.58901509 + 2.142857143 x = -0.58901509 + 2.142857143 Combine like terms: -0.58901509 + 2.142857143 = 1.553842053 x = 1.553842053 Simplifying x = 1.553842053

Solution

The solution to the problem is based on the solutions from the subproblems. x = {2.731872233, 1.553842053}

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