The following are stories and questions about them. Each story is followed by a question and answer to a given question.

Story: The radian is the standard unit of angular measure, used in many areas of mathematics. The length of an arc of a unit circle is numerically equal to the measurement in radians of the angle that it s; one radian is just under 57.3 degrees (expansion at ). The unit was formerly an SI supplementary unit, but this category was abolished in 1995 and the radian is now considered an SI derived unit. 

Separately, the SI unit of solid angle measurement is the steradian. 

The radian is represented by the symbol rad. An alternative symbol is , the superscript letter c (for "circular measure"), the letter r, or a superscript , but these symbols are infrequently used as it can be easily mistaken for a degree symbol (°) or a radius (r). So, for example, a value of 1.2 radians could be written as 1.2 rad, 1.2 r, 1.2, or 1.2, or 1.2. 

Radian describes the plane angle subtended by a circular arc as the length of the arc divided by the radius of the arc. One radian is the angle subtended at the center of a circle by an arc that is equal in length to the radius of the circle. More generally, the magnitude in radians of such a subtended angle is equal to the ratio of the arc length to the radius of the circle; that is, , where "θ" is the subtended angle in radians, "s" is arc length, and "r" is radius. Conversely, the length of the enclosed arc is equal to the radius multiplied by the magnitude of the angle in radians; that is, .

Question: What symbol represents the radian?
Answer:rad.

Question: Are there any others?
Answer:yes

Question: Are they frequently used?
Answer:no

Question: Why?
Answer:but these symbols are infrequently used as it can be easily mistaken for a degree symbol (°) or a radius (r

Question: What does the term describe?
Answer:the plane angle subtended by a circular arc

Question: Is it used in many areas?
Answer:yes

Question: What is equal to the measurement in radians?
Answer:The length of an arc of a unit circle

Question: What was the units former classification?
Answer:an SI supplementary unit

Question: Does that category still exist?
Answer:no

Question: What happened to it?
Answer:it was abolished

Question: When?
Answer:1995

Question: What is the category it is in now?
Answer:an SI derived unit.

Question: What is a steradian?
Answer:
the SI unit of solid angle measurement