Course: Trigonometry
Topic: Circular Functions
Subtopic: Radian Measure


To date we have only measured angles in degrees, but today we will introduce a new angle measure called radian measure. It is based on irrational number pi (π) rather than the number 360. Notation caution: Radians don't (usually) have a symbol indicating radian measure, so leaving the degree mark off looks like radians! For example, tan(45) means the tangent of 45 radians. To indicate the tangent of 45 degrees you must write tan(45°). tan(45)≠tan(45°) ... check this electronically noting that the tan(45) must be evaluated in radian mode and tan(45°) must be evaluated in degree mode so set your device/software accordingly.


By the end of this topic you should know and be prepared to be tested on:


Terms you should be able to define: pi (`pi`), radian, unitless measure

Mini-Lectures and Examples

STUDY: Radian Measure & Arc Length & Area of a Sector

Supplementary Resources (recommended)

Everyone should READ THIS ARTICLE: Why Use Radian Measure?

You must be able to use a calculator (handheld or software) to work with angles in radians. If you have a handheld graphing calculator then these sites may help you learn about working in radian mode:

Supplemental Resources (optional)

Videos: Radian Measure
Examples:  Converting Angles in Degree Measure to Radian Measure
Examples:  Convert Angles in Radian Measure to Degree Measure
Examples:  Determining Coterminal Angles in Radian Measure

rev. 2020-10-10