A medical researcher says that at least 20% of U.S. adults are smokers. In a random sample of 2000 U.S. adults, 18.5% say that they are smokers. At a = 0.05, test the claim.

Explanation:

Part A:  Let's say we have a figure having a set of points, but the set of all points on this figure are not on a plane (flat surface), it means they cannot be of the same distance from a fixed point because as this set of points aren't on a plane there would be irregularities in their distances from the fixed point.

A more proper definition could be: A circle is the set of all points on a plane that has the same distance from one fixed point.

Part B: A good example is the term radius (radius of a circle), which refers to any line or interval that joins or touches a point on the circle to the center.