100=10*(3.14*(r*r))

Solution for 100=10*(3.14*(r*r)) equation:

Simplifying
100 = 10(3.14(r * r))

Multiply r * r
100 = 10(3.14(r2))

Remove parenthesis around (3.14r2)
100 = 10 * 3.14r2

Multiply 10 * 3.14
100 = 31.4r2

Solving
100 = 31.4r2

Solving for variable 'r'.

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

Add '-31.4r2' to each side of the equation.
100 + -31.4r2 = 31.4r2 + -31.4r2

Combine like terms: 31.4r2 + -31.4r2 = 0.0
100 + -31.4r2 = 0.0

Add '-100' to each side of the equation.
100 + -100 + -31.4r2 = 0.0 + -100

Combine like terms: 100 + -100 = 0
0 + -31.4r2 = 0.0 + -100
-31.4r2 = 0.0 + -100

Combine like terms: 0.0 + -100 = -100
-31.4r2 = -100

Divide each side by '-31.4'.
r2 = 3.184713376

Simplifying
r2 = 3.184713376

Take the square root of each side:
r = {-1.784576526, 1.784576526}