Math Thematics Book 2

Module 2 Section 1

Exploration 1: Measuring Angles

Rays and Angles

A ray is part of a line. It starts at an endpoint and goes on forever in one direction. Always write the endpoint of the line first, ray BC.

An angle is formed by two rays with a common endpoint called the vertex of the angle. The angle below, formed by BA and BC, can be called ÐABC, ÐCBA, or ÐB. Always write the vertex as the middle letter.

Measuring and Classifying Angles

The measure of an angle is the amount of rotation between its two rays. An angle is measured in units called degrees. There are 360º in a complete rotation, so one degree is 1/360 of a complete rotation.

An angle can be classified by its measure.

Exploration 2: Angle Relationships

Supplementary and Complementary

Two angles are supplementary angles if the sum of the measures of the angles is 180º. ÐP and ÐQ below are supplementary angles.

Two angles are complementary angles if the sum of the measures of the angles is 90º. ÐX and ÐY below are complementary angles.