2r^2-4r=15

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Solution for 2r^2-4r=15 equation:


Simplifying
2r2 + -4r = 15

Reorder the terms:
-4r + 2r2 = 15

Solving
-4r + 2r2 = 15

Solving for variable 'r'.

Reorder the terms:
-15 + -4r + 2r2 = 15 + -15

Combine like terms: 15 + -15 = 0
-15 + -4r + 2r2 = 0

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

Divide each side by '2'.
-7.5 + -2r + r2 = 0

Move the constant term to the right:

Add '7.5' to each side of the equation.
-7.5 + -2r + 7.5 + r2 = 0 + 7.5

Reorder the terms:
-7.5 + 7.5 + -2r + r2 = 0 + 7.5

Combine like terms: -7.5 + 7.5 = 0.0
0.0 + -2r + r2 = 0 + 7.5
-2r + r2 = 0 + 7.5

Combine like terms: 0 + 7.5 = 7.5
-2r + r2 = 7.5

The r term is -2r.  Take half its coefficient (-1).
Square it (1) and add it to both sides.

Add '1' to each side of the equation.
-2r + 1 + r2 = 7.5 + 1

Reorder the terms:
1 + -2r + r2 = 7.5 + 1

Combine like terms: 7.5 + 1 = 8.5
1 + -2r + r2 = 8.5

Factor a perfect square on the left side:
(r + -1)(r + -1) = 8.5

Calculate the square root of the right side: 2.915475947

Break this problem into two subproblems by setting 
(r + -1) equal to 2.915475947 and -2.915475947.

Subproblem 1

r + -1 = 2.915475947 Simplifying r + -1 = 2.915475947 Reorder the terms: -1 + r = 2.915475947 Solving -1 + r = 2.915475947 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '1' to each side of the equation. -1 + 1 + r = 2.915475947 + 1 Combine like terms: -1 + 1 = 0 0 + r = 2.915475947 + 1 r = 2.915475947 + 1 Combine like terms: 2.915475947 + 1 = 3.915475947 r = 3.915475947 Simplifying r = 3.915475947

Subproblem 2

r + -1 = -2.915475947 Simplifying r + -1 = -2.915475947 Reorder the terms: -1 + r = -2.915475947 Solving -1 + r = -2.915475947 Solving for variable 'r'. Move all terms containing r to the left, all other terms to the right. Add '1' to each side of the equation. -1 + 1 + r = -2.915475947 + 1 Combine like terms: -1 + 1 = 0 0 + r = -2.915475947 + 1 r = -2.915475947 + 1 Combine like terms: -2.915475947 + 1 = -1.915475947 r = -1.915475947 Simplifying r = -1.915475947

Solution

The solution to the problem is based on the solutions from the subproblems. r = {3.915475947, -1.915475947}

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