Question

open - ended
what is the difference between radius and diameter of a circle?

Explanation:

The radius is the distance from the center of a circle to any point on its circumference. The diameter is the distance across the circle through its center. So, the diameter is twice the radius ($d = 2r$), and the radius is half of the diameter ($r=\frac{d}{2}$).

Answer:

The radius ($r$) is the distance from the center to the circumference. The diameter ($d$) is twice the radius ($d = 2r$), spanning the circle through the center.