**Introduction to Lines and Angles**

*There is geometry in the humming of the strings*.

—Pythagoras (580–500 B.C.)

Let's learn the lingo of lines and angles.

**Angles can be **measured with a tool called the **protractor.**

A **line** is 180 degrees and can be thought of as a perfectly flat angle.

A **ray** is part of a line that has one endpoint. It extends indefinitely in one direction.

An **acute angle** is less than 90°.

A **right angle** equals 90°.

A **straight line** is 180°.

An **obtuse angle** is greater that 90°.

Two angles are **supplementary** if they add to 180°.

Two angles are **complementary** if they add to 90°.

If you **bisect** an angle, you cut it exactly in half. This forms **congruent** angles, which have the same measure.

When two lines intersect, four angles are formed. The sum of these angles is 360 degrees.

When two lines are perpendicular to each other, their intersection forms four 90-degree angles, which are also called **right angles**. Right angles are identified by little boxes at the intersection of the angle's arms.

When lines are **parallel,** they never meet.

A **transversal** is a line that intersects two or more other lines. In the following figure, angles 1, 3, 5, and 7 are equal. Angles 2, 4, 6, and 8 are equal. Angles 1 and 7 and 2 and 8 are called **alternate exterior angles**.Angles 3 and 5 and 4 and 6 are called **alternate interior angles**.

**Vertical angles** are the angles across from each other that are formed by intersecting lines. Vertical lines are always equal. In the following figure, angles 1 and 3 are equal vertical angles and angles 2 and 4 are equal vertical angles.

Find practice problems and solutions for these concepts at Lines and Angles Practice Questions.

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