- the fact that the sum of the interior angles of any triangle is π radians (180 degrees), and
- Pythagoras' Theorem.

A student wishing to read this Chapter of the course should be familiar with the idea and geometrical meaning of angles and angular measure. This material can be reviewed, if needed, by reading (or re-reading) Chapter 1 of the course. Two other specific pieces of background (which many students are already familiar with) are

Both of these are explained/reviewed in the files accessible by clicking the relevant link below.

The main topic of this chapter is the basic geometrical meaning of the six trigonometric functions as ratios of the sides of right angle triangles. Since the non-right angles in a right angle triangle are necessarily smaller than π/2 radians (90 degrees) this approach provides the natural basic definitions of the trig functions for angles lying between 0 and π/2 radians, i.e., for first quadrant values of the angle. The geometrical definitions discussed here provide the most basic meanings of the trig functions, and a familiarity with these definitions is crucial to acquiring a good intuitive understanding of the behavior of these functions.

**LINKS FOR THE NEW CHAPTER 2 TOPICS:**

*Click on the link for the appropriate topic below for more information
on that topic. In general, it would be a good idea to go through the
material in the order listed, unless you are returning to this page
for a refresher on a particular topic (or topics) or are already
familiar with some of the topics from previous review.
*