Alicia Young, age 15, of Rock Island, I11., for her question:

WHAT IS A RADIAN?

A radian is a metric unit used to measure angles. Engineers and scientists often measure angles in radians because the unit simplifies many of their calculations.

Navigators, surveyors and most other people measure angles in degrees. One radian equals an angle of 57.29578 degrees.

An angle of one radian is formed between two radii of a circle if they mark off an arc equal to the length of the radius of the circle. Circles and arcs may represent angles and may be measured in radians.

As an example, the circumference of a circle equals two times pi, times the radius of the circle. Thus, there are two pi radians in a circle. An arc of half a circle equals pi radians.

In calculus and advanced mathematics, the radian system measures angles. In this system, one revolution, or 360 degrees, equals two pi radians.