81650.99=2*3.14*r^2+2*3.14*r*100.2

Solution for 81650.99=2*3.14*r^2+2*3.14*r*100.2 equation:

Simplifying
81650.99 = 2 * 3.14r2 + 2 * 3.14r * 100.2

Multiply 2 * 3.14
81650.99 = 6.28r2 + 2 * 3.14r * 100.2

Reorder the terms for easier multiplication:
81650.99 = 6.28r2 + 2 * 3.14 * 100.2r

Multiply 2 * 3.14
81650.99 = 6.28r2 + 6.28 * 100.2r

Multiply 6.28 * 100.2
81650.99 = 6.28r2 + 629.256r

Reorder the terms:
81650.99 = 629.256r + 6.28r2

Solving
81650.99 = 629.256r + 6.28r2

Solving for variable 'r'.

Reorder the terms:
81650.99 + -629.256r + -6.28r2 = 629.256r + -629.256r + 6.28r2 + -6.28r2

Combine like terms: 629.256r + -629.256r = 0.000
81650.99 + -629.256r + -6.28r2 = 0.000 + 6.28r2 + -6.28r2
81650.99 + -629.256r + -6.28r2 = 6.28r2 + -6.28r2

Combine like terms: 6.28r2 + -6.28r2 = 0.00
81650.99 + -629.256r + -6.28r2 = 0.00

Begin completing the square.  Divide all terms by
-6.28 the coefficient of the squared term: 

Divide each side by '-6.28'.
-13001.75 + 100.2r + r2 = 0

Move the constant term to the right:

Add '13001.75' to each side of the equation.
-13001.75 + 100.2r + 13001.75 + r2 = 0 + 13001.75

Reorder the terms:
-13001.75 + 13001.75 + 100.2r + r2 = 0 + 13001.75

Combine like terms: -13001.75 + 13001.75 = 0.00
0.00 + 100.2r + r2 = 0 + 13001.75
100.2r + r2 = 0 + 13001.75

Combine like terms: 0 + 13001.75 = 13001.75
100.2r + r2 = 13001.75

The r term is 100.2r.  Take half its coefficient (50.1).
Square it (2510.01) and add it to both sides.

Add '2510.01' to each side of the equation.
100.2r + 2510.01 + r2 = 13001.75 + 2510.01

Reorder the terms:
2510.01 + 100.2r + r2 = 13001.75 + 2510.01

Combine like terms: 13001.75 + 2510.01 = 15511.76
2510.01 + 100.2r + r2 = 15511.76

Factor a perfect square on the left side:
(r + 50.1)(r + 50.1) = 15511.76

Calculate the square root of the right side: 124.546216321

Break this problem into two subproblems by setting 
(r + 50.1) equal to 124.546216321 and -124.546216321.

Subproblem 1
r + 50.1 = 124.546216321

Simplifying
r + 50.1 = 124.546216321

Reorder the terms:
50.1 + r = 124.546216321

Solving
50.1 + r = 124.546216321

Solving for variable 'r'.

Move all terms containing r to the left, all other terms to the right.

Add '-50.1' to each side of the equation.
50.1 + -50.1 + r = 124.546216321 + -50.1

Combine like terms: 50.1 + -50.1 = 0.0
0.0 + r = 124.546216321 + -50.1
r = 124.546216321 + -50.1

Combine like terms: 124.546216321 + -50.1 = 74.446216321
r = 74.446216321

Simplifying
r = 74.446216321

Subproblem 2
r + 50.1 = -124.546216321

Simplifying
r + 50.1 = -124.546216321

Reorder the terms:
50.1 + r = -124.546216321

Solving
50.1 + r = -124.546216321

Solving for variable 'r'.

Move all terms containing r to the left, all other terms to the right.

Add '-50.1' to each side of the equation.
50.1 + -50.1 + r = -124.546216321 + -50.1

Combine like terms: 50.1 + -50.1 = 0.0
0.0 + r = -124.546216321 + -50.1
r = -124.546216321 + -50.1

Combine like terms: -124.546216321 + -50.1 = -174.646216321
r = -174.646216321

Simplifying
r = -174.646216321

Solution
The solution to the problem is based on the solutions
from the subproblems.
r = {74.446216321, -174.646216321}