Angles

Study Sheet

Angles

Measuring and classifying turns

Naming Angles

Concept
The Big Idea
55°

An angle measures a turn. A quarter turn is 90∘90^\circ (a right angle), and a half turn is 180∘180^\circ (a straight angle).

Tip
The three names

Less than 90∘90^\circ is acute. Exactly 90∘90^\circ is right. Between 90∘90^\circ and 180∘180^\circ is obtuse.

Example
This angle
55°

It opens less than a quarter turn, and it measures 55∘55^\circ, so it is acute.

In Plain Terms & More Examples

Concept
In plain terms

An angle measures how far something turns. Compare it to a right angle (90∘90^\circ): smaller is acute, bigger (up to 180∘180^\circ) is obtuse.

Example
A 120∘120^\circ angle

It is more than 90∘90^\circ but less than 180∘180^\circ, so it is obtuse.

Example
A 30∘30^\circ angle

It is less than 90∘90^\circ, so it is acute.