# Cosine Function Introduction

In the circle above (which has radius 1), we see a radius from the point to point A, and half of a chord, from the point A to the y axis. As we change the value of the angle , the length of the half chord changes. The study of the length of these half chords is the origin of the cosine function.

When , we see that the horizontal half chord has length 0.87.

We notice that in the first quadrant, this length is the same as the x coordinate of the point A.

We define the cosine of the angle as the x coordinate of the point A. Therefore, , (2 s.f.). Also, (2 s.f.).

The graph on the right maps the value of to the x coordinate of the point A. Notice the point .

Move the slider to see how cosine changes (the x coordinate of point A) as the angle changes.

## Quick Comprehension Exercises:

You may use the applet above to help answer these questions.

1. What is the value of cos(60°)?
2. What is the value of cos(144°), correct to two decimal places?
3. Is the cosine of 272° positive or negative? 4. In which quadrant is the angle 195°?
5. In which two quadrants is the value of cos(α°) negative?