10000=3.14*r^2(20)

Solution for 10000=3.14*r^2(20) equation:

Simplifying
10000 = 3.14r2(20)

Reorder the terms for easier multiplication:
10000 = 3.14 * 20r2

Multiply 3.14 * 20
10000 = 62.8r2

Solving
10000 = 62.8r2

Solving for variable 'r'.

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

Add '-62.8r2' to each side of the equation.
10000 + -62.8r2 = 62.8r2 + -62.8r2

Combine like terms: 62.8r2 + -62.8r2 = 0.0
10000 + -62.8r2 = 0.0

Add '-10000' to each side of the equation.
10000 + -10000 + -62.8r2 = 0.0 + -10000

Combine like terms: 10000 + -10000 = 0
0 + -62.8r2 = 0.0 + -10000
-62.8r2 = 0.0 + -10000

Combine like terms: 0.0 + -10000 = -10000
-62.8r2 = -10000

Divide each side by '-62.8'.
r2 = 159.2356688

Simplifying
r2 = 159.2356688

Take the square root of each side:
r = {-12.618861629, 12.618861629}