How do I find the radius
Radius is half times the diameter. When the surface area or the volume is known, the radius of a circle or a sphere can be calculated.
Radius of a circle
This is the formula to calculate the radius of a circle, with A being the surface area of the circle: Radius circle
Radius of a sphere
This is the formula to calculate the radius of a sphere, with A being the surface area of the sphere and V being the volume of the sphere: Radius sphere
Exercises
  1. The surface area of a circle is 23. What is the radius?
  2. How large is the area of a circle with diameter 2?
  3. What is the volume of a sphere with radius 8?
  4. A sphere has a volume of 15. How large is the area?
  5. The volume of a sphere is 45. What should the radius of a circle be, if the surface areas are equal?
Solutions
  1. 2,71
  2. 3,14
  3. 2144,7