Lines, Rays, and Angles

Basic Geometric Figures

| Figure | Description | Symbol | |--------|-------------|--------| | Point | An exact location in space | Point $A$ | | Line | Extends forever in both directions | $\overleftrightarrow{AB}$ | | Line segment | Part of a line with two endpoints | $\overline{AB}$ | | Ray | Part of a line with one endpoint, extends forever in one direction | $\overrightarrow{AB}$ |

Types of Lines

• Parallel lines: Lines that never intersect (like railroad tracks) ∥
• Perpendicular lines: Lines that intersect at a right angle (90°) ⊥
• Intersecting lines: Lines that cross at a point

Angles

An angle is formed by two rays sharing a common endpoint (called the vertex).

Types of Angles

| Type | Measure | Description | |------|---------|-------------| | Right angle | Exactly $90°$ | Looks like a corner of a square | | Acute angle | Less than $90°$ | "Small" angle | | Obtuse angle | Between $90°$ and $180°$ | "Wide" angle | | Straight angle | Exactly $180°$ | Looks like a straight line |

Measuring Angles with a Protractor

1. Place the center of the protractor on the vertex
2. Line up one ray with the $0°$ line
3. Read where the other ray crosses the protractor

Symmetry

A shape has a line of symmetry if it can be folded in half and both sides match perfectly.

• A square has 4 lines of symmetry
• A rectangle has 2 lines of symmetry
• A circle has infinite lines of symmetry

Practice: Find all the right angles in your classroom!