Diameter (from the Greek διάμετρος "diameter, the diameter") is a segment that connects two points on the circle or sphere passing through the center of the circle or sphere. Diameter is also called the length of this segment. Radius (from Latin radius "ray, spoke of wheel") is a segment that connects the center of a circle or sphere to any point on that circle or sphere, the radius is also called the length of this segment.
The radius is denoted by the letter r, the diameter with the letter d. By definition, the radius is half the diameter, and the diameter equal to twice the radius. Accordingly, d=2r, r=d/2. So, in order to find the radius, knowing the diameter, divide the diameter by two.
Example. Diameter of circle d is 8. What is the radius r? Solution: r=d/2, so to find the radius, you have a diameter value of 8 divided by two. 8/2=4. Answer: r=4, the radius is equal to four.
If you are looking for the length of the radius or diameter, remember that length cannot be a negative number. So if you come to the formula d=2r= √x (square root of x), and x is, for example 16, the diameter d=±4,and radius r=±2. Since length cannot be a negative number, the answer is: the diameter is four, the radius is equal to two.
Interesting fact that the anatomy is also found in the word "radius", it means one of the bones of the forearm, the radius bone (located laterally and slightly anterior to the ulna). And yet the word radius is the value it takes its origins in ancient Rome is the name of the Roman short sword used by the legionaries to defend. Legionnaire said: "this is where I Rome!" – traced on the ground with this sword lane and defended themselves to the last.