Arc LengthA radian is another unit of measurement for angles just like degrees. Radians are especially convenient for measuring angles that have to do with circles. There are 360 degrees in a circle and 90 degrees in a right angle. How many radians are in a circle and how many radians make up a right angle?Lesson, videos, exercises, and text from CK-12. Additional resources available at this site.

- Mathematics > General
- Mathematics > Geometry

- Grade 9
- Grade 10

Reason abstractly and quantitatively.

Construct viable arguments and critique the reasoning of others.

Use appropriate tools strategically.

Attend to precision.

Derive using similarity the fact that the length of the arc intercepted by an angle is proportional to the radius, and define the radian measure of the angle as the constant of proportionality; derive the formula for the area of a sector.