Radius From the Surface
Part of the Surface Area & Volume section of Coddy's Geometry journey. Lesson 50 of 62.
Back from a sphere's surface area to its radius is two moves: divide by 4π to get r², then take the positive square root.
This sphere has surface area 64π. The rule says 64×pi = 4×pi×r^2, so r^2 = 16 and the radius is 4. Check: 4×pi × 4^2 = 64×pi.
On the figure the radius is marked ?. Tap it and pick Surface area of a sphere: the engine runs the rule backwards and keeps the positive root.
Challenge
The sphere's surface area is 196π. Find its radius, marked ?, then press Check.
Try it yourself
Find the marked part of the figure
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Surface Area & Volume
1Solids and Their Parts
Faces, Edges and VerticesPrisms and CylindersPyramids and ConesHeight or Slant Height?Spheres and HemispheresRecap: Parts of Solids4Pyramids
Volume of a PyramidThe Slant HeightHeight From the SlantHeight From the VolumeRecap: Pyramids7Surface Area of Prisms
Nets and Surface AreaVolume or Surface Area?A Missing Edge AgainTriangular PrismsThe Hidden Side FirstRecap: Prism Surfaces10Spheres Outside
Surface of a SphereSurface of a HemisphereRadius From the SurfaceSurface to VolumeRecap: Spheres Outside2Volume of Prisms
Volume of a CuboidVolume of a PrismAny Triangle EndA Missing EdgeRecap: Prism Volume3Cylinders
Volume of a CylinderHeight From the VolumeRadius From the VolumeEnough to Find It?Recap: Cylinders6Spheres and Hemispheres
Volume of a SphereHalf a SphereRadius From the VolumeSphere in a CylinderRecap: Spheres9Pyramids and Cones Outside
Surface of a PyramidSlant Height FirstSurface of a ConeThe Cone: Slant FirstRecap: Pyramid Surfaces