5k^2-20k=81

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Solution for 5k^2-20k=81 equation:


Simplifying
5k2 + -20k = 81

Reorder the terms:
-20k + 5k2 = 81

Solving
-20k + 5k2 = 81

Solving for variable 'k'.

Reorder the terms:
-81 + -20k + 5k2 = 81 + -81

Combine like terms: 81 + -81 = 0
-81 + -20k + 5k2 = 0

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

Divide each side by '5'.
-16.2 + -4k + k2 = 0

Move the constant term to the right:

Add '16.2' to each side of the equation.
-16.2 + -4k + 16.2 + k2 = 0 + 16.2

Reorder the terms:
-16.2 + 16.2 + -4k + k2 = 0 + 16.2

Combine like terms: -16.2 + 16.2 = 0.0
0.0 + -4k + k2 = 0 + 16.2
-4k + k2 = 0 + 16.2

Combine like terms: 0 + 16.2 = 16.2
-4k + k2 = 16.2

The k term is -4k.  Take half its coefficient (-2).
Square it (4) and add it to both sides.

Add '4' to each side of the equation.
-4k + 4 + k2 = 16.2 + 4

Reorder the terms:
4 + -4k + k2 = 16.2 + 4

Combine like terms: 16.2 + 4 = 20.2
4 + -4k + k2 = 20.2

Factor a perfect square on the left side:
(k + -2)(k + -2) = 20.2

Calculate the square root of the right side: 4.494441011

Break this problem into two subproblems by setting 
(k + -2) equal to 4.494441011 and -4.494441011.

Subproblem 1

k + -2 = 4.494441011 Simplifying k + -2 = 4.494441011 Reorder the terms: -2 + k = 4.494441011 Solving -2 + k = 4.494441011 Solving for variable 'k'. Move all terms containing k to the left, all other terms to the right. Add '2' to each side of the equation. -2 + 2 + k = 4.494441011 + 2 Combine like terms: -2 + 2 = 0 0 + k = 4.494441011 + 2 k = 4.494441011 + 2 Combine like terms: 4.494441011 + 2 = 6.494441011 k = 6.494441011 Simplifying k = 6.494441011

Subproblem 2

k + -2 = -4.494441011 Simplifying k + -2 = -4.494441011 Reorder the terms: -2 + k = -4.494441011 Solving -2 + k = -4.494441011 Solving for variable 'k'. Move all terms containing k to the left, all other terms to the right. Add '2' to each side of the equation. -2 + 2 + k = -4.494441011 + 2 Combine like terms: -2 + 2 = 0 0 + k = -4.494441011 + 2 k = -4.494441011 + 2 Combine like terms: -4.494441011 + 2 = -2.494441011 k = -2.494441011 Simplifying k = -2.494441011

Solution

The solution to the problem is based on the solutions from the subproblems. k = {6.494441011, -2.494441011}

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