Radius From the Surface

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 $64\cdot \pi = 4\cdot \pi\cdot {r}^{2}$, so ${r}^{2} = 16$ and the radius is 4. Check: $4\cdot \pi\cdot {4}^{2} = 64\cdot \pi$.

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.