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, . QUESTIONS:

Q1:
1. What symbol is used to signify the radian?
2. Which symbol is associated with the radian?
3. 
Q2:
1. Do symbols other than rad represent the radian?
2. Are there symbols besides the rad that are used to designate the radian?
3. 
Q3:
1. Are alternative symbols for the radian frequently used?
2. Are c and r often used to designate the radian?
3. 
Q4:
1. Why aren't c and r often used to represent the radian?
2. What makes it so that c and r are infrequently used to designate the radian?
3. 
Q5:
1. What is a radian?
2. What does the term radian mean?
3. 
Q6:
1. Are radians used in many areas?
2. Do many fields make use of the radian?
3. 
Q7:
1. What is the same as the measurement in radians?
2. What equals measurement in radians?
3. 
Q8:
1. How were radians formery classified?
2. What did radians used to be categorized as?
3. 
Q9:
1. Is the category SI supplementary unit still in use?
2. Does the SI supplementary unit classification get used today?
3. 
Q10:
1. What happened to the category of SI supplementary unit?
2. What became of SI supplementary unit classification?
3. 
Q11:
1. When was SI supplementary unit abolished?
2. In what year was SI supplementary unit done away with?
3. 
Q12:
1. What category are radians in now?
2. How are radians currently classified?
3. 
Q13:
1. How can a steradian be defined?
2. What is the definition of a steradian?
3. 
