21x^2-9x-4=0

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Solution for 21x^2-9x-4=0 equation:


Simplifying
21x2 + -9x + -4 = 0

Reorder the terms:
-4 + -9x + 21x2 = 0

Solving
-4 + -9x + 21x2 = 0

Solving for variable 'x'.

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

Divide each side by '21'.
-0.1904761905 + -0.4285714286x + x2 = 0

Move the constant term to the right:

Add '0.1904761905' to each side of the equation.
-0.1904761905 + -0.4285714286x + 0.1904761905 + x2 = 0 + 0.1904761905

Reorder the terms:
-0.1904761905 + 0.1904761905 + -0.4285714286x + x2 = 0 + 0.1904761905

Combine like terms: -0.1904761905 + 0.1904761905 = 0.0000000000
0.0000000000 + -0.4285714286x + x2 = 0 + 0.1904761905
-0.4285714286x + x2 = 0 + 0.1904761905

Combine like terms: 0 + 0.1904761905 = 0.1904761905
-0.4285714286x + x2 = 0.1904761905

The x term is -0.4285714286x.  Take half its coefficient (-0.2142857143).
Square it (0.04591836735) and add it to both sides.

Add '0.04591836735' to each side of the equation.
-0.4285714286x + 0.04591836735 + x2 = 0.1904761905 + 0.04591836735

Reorder the terms:
0.04591836735 + -0.4285714286x + x2 = 0.1904761905 + 0.04591836735

Combine like terms: 0.1904761905 + 0.04591836735 = 0.23639455785
0.04591836735 + -0.4285714286x + x2 = 0.23639455785

Factor a perfect square on the left side:
(x + -0.2142857143)(x + -0.2142857143) = 0.23639455785

Calculate the square root of the right side: 0.486204235

Break this problem into two subproblems by setting 
(x + -0.2142857143) equal to 0.486204235 and -0.486204235.

Subproblem 1

x + -0.2142857143 = 0.486204235 Simplifying x + -0.2142857143 = 0.486204235 Reorder the terms: -0.2142857143 + x = 0.486204235 Solving -0.2142857143 + x = 0.486204235 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '0.2142857143' to each side of the equation. -0.2142857143 + 0.2142857143 + x = 0.486204235 + 0.2142857143 Combine like terms: -0.2142857143 + 0.2142857143 = 0.0000000000 0.0000000000 + x = 0.486204235 + 0.2142857143 x = 0.486204235 + 0.2142857143 Combine like terms: 0.486204235 + 0.2142857143 = 0.7004899493 x = 0.7004899493 Simplifying x = 0.7004899493

Subproblem 2

x + -0.2142857143 = -0.486204235 Simplifying x + -0.2142857143 = -0.486204235 Reorder the terms: -0.2142857143 + x = -0.486204235 Solving -0.2142857143 + x = -0.486204235 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '0.2142857143' to each side of the equation. -0.2142857143 + 0.2142857143 + x = -0.486204235 + 0.2142857143 Combine like terms: -0.2142857143 + 0.2142857143 = 0.0000000000 0.0000000000 + x = -0.486204235 + 0.2142857143 x = -0.486204235 + 0.2142857143 Combine like terms: -0.486204235 + 0.2142857143 = -0.2719185207 x = -0.2719185207 Simplifying x = -0.2719185207

Solution

The solution to the problem is based on the solutions from the subproblems. x = {0.7004899493, -0.2719185207}

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