To measure angles, it is convenient to use the circumference of a circle. A full revolution (counterclockwise) corresponds to 360 degrees, a half revolution to 180 degrees, a quarter revolution to 90 degrees, and so on.

Recall that in a coordinate system there are four quadrants, numbered I, II, III, and IV. The diagram below shows which angles betwee 0 degrees and 360 degrees lie in each of the four quadrants.

Below is a right triangle, one of whose angles is labeled . The three sides
of the triangle are the **hypotenuse, **the **opposite side** (the
side opposite the angle ), and the** adjacent** **side** (the
side adjacent to the angle ). Using the lengths of these three sides,
we can form three ratios that define three trigonometric functions of the
acute angle .

We can now use this information to investigate these three trigonometric functions.

Click **HERE** to investigate the trigonometric
functions.