Set up the equation so the area ("a") equals πr2 (i.e., pi x r squared). This puts the equation in a solvable format in order to determine the radius ("r").
Divide both sides of the equation by π (pi). This will have you dividing "a" by 3.14 and will cancel out π on the other side.
Solve for "r" by taking the square root of both sides of the equation. The number you have determined by dividing "a" by π will be the squared amount of "r." By calculating the square root, you will be left with "r" -- the radius.
Set up the equation so that the circumference ("c") equals 2πr (i.e., 2 x pi x r). This leaves you with an equation that you can solve for "r."
Divide both sides of the equation by 2. This will leave you with πr on one side and a number on the other side.
Divide both sides of the equation by π (3.14) to eliminate π on one side. You will now have "r" on one side and a number on the other. The number is the radius.
Set up the equation so that the surface area equals 4πr2 (i.e., 4 x pi x r squared) and solve for "r."
Divide both sides by 4, which will leave you with "πr2" on one side. Now divide both sides by π (3.14).
Take the square root of both sides of the equation, which will leave you with the radius of the sphere.
Set up the equation so that the radius equals the length of any side divided by (2sin x (π/the number of sides). This makes solving for the radius a matter of working with the formula.
Determine the length of the side to fill in the top of the right side of the equation.
Divide π by the number of sides. For example, a pentagon has five sides so you would divide 3.14 by 5. Multiple this answer by sin and by 2.
Take the length of the side and divide it by the total you received when you worked the entire 2sin x (π/the number of sides). This gives you the radius between the two points.