Evaluating Trig. Functions Using Reference Angle

The following figures give examples of the standard angle and the reference angle for the different quadrants. Scroll down the page for more examples and solutions.

What is The Reference Angle Theorem?
The Reference Angle Theorem states that
To find the value of a trigonometric function of any angle t:

1. Determine the function value for the associated reference angle t’.
2. Depending on the quadrant in which t lies, the answer will be either be + or -.

How to evaluate trig functions using reference angles?

1. Find the reference angle for the given angle.
2. Rewrite the original trig function with the reference angle.
3. Affix the appropriate sign to the trig function (+ or -), depending which quadrant the angle terminates in.
4. Evaluate the trig function.