Review: Classify and measure angles

An angle is formed between two lines, rays, or segments whenever they intersect. We can think of an angle as a turn from one object to the other.

The most important angle in geometry is called a right angle, and represents a quarter of a turn around a circle. When two objects form a right angle, we say they are perpendicular. We draw a right angle using a small square rather than a circular arc.

We draw all other angles with a circular arc. An angle that is smaller than a right angle is called an acute angle.

Two right angles together form a straight angle:

Four right angles is the same as two straight angles, making a full revolution:

An angle that is larger than a right angle but smaller than a straight angle is called an obtuse angle.

We met the last kind of angle in the previous lesson - a reflex angle is larger than a straight angle, but smaller than a full revolution.

Angles are a measure of turning. All angles can be compared to a right angle, representing a quarter turn.

Examples

Example 1

Select the obtuse angle:

A

B

C

D

Worked Solution

Create a strategy

Use this image to help you:

Rotate each angle so that one arm lies over the start.

Apply the idea

The answer is Option C.

Watch question walkthrough

We divide a full revolution up into 360 small turns called degrees, and write the unit using a small circle, like this: 360\degree.

Since 90 is one quarter of 360, we know that a right angle is exactly 90\degree. This circle has markings every 45\degree:

A full revolution is made up of 360 degrees, a single degree is written 1\degree.

Examples

Example 2

Select the angle that is closest to 120\degree:

A

B

C

D

Worked Solution

Create a strategy

Use this image to help you:

Rotate each angle so that one arm lies over the start.

Apply the idea

The answer is Option C.

Watch question walkthrough