Answer: multiply the diameter by 1/2 to get the radius
Step-by-step explanation: A diameter is a line across a circle at which its endpoints lie on the circle and travels through the center of the said circle, cutting the circle perfectly in half. The radius is the measurement from the sender of the circle to any given point on the circle and is always equidistant and congruent to other radii of the same circle. For this reason, the radius is always half the length of the diameter. Thus, to define the relationship between the two, multiplying the diameter by 1/2 would be the same as diving it by 2, giving you the radius of the circle.