Concept Check Match each angle in Column I with its reference angle in Column II. Choices may be used once, more than once, or not at all. See Example 1. I. II. 5. A. 45° 6. B. 60° 7. -135° C. 82° 8. D. 30° 9. E. 38° 10. F. 32°

Key Concepts

Reference Angle

A reference angle is the acute angle formed by the terminal side of an angle in standard position and the x-axis. It is always measured as a positive angle and is used to simplify the calculation of trigonometric functions. For angles greater than 180° or less than 0°, the reference angle is found by subtracting or adding to 180° or 360°, respectively, to find the corresponding acute angle.

Angle Measurement

Angles can be measured in degrees or radians, with degrees being the more common unit in basic trigonometry. A full circle is 360 degrees, and angles can be positive (counterclockwise) or negative (clockwise). Understanding how to convert between these measurements and how to interpret negative angles is essential for finding reference angles.

Quadrants of the Coordinate Plane

The coordinate plane is divided into four quadrants, each defined by the signs of the x and y coordinates. The location of an angle determines its reference angle and the corresponding trigonometric values. Knowing which quadrant an angle lies in helps in determining the correct reference angle and its associated sine, cosine, and tangent values.