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. Here are two:
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. Here are two:
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. Here are two:
Angles are a measure of turning. All angles can be compared to a right angle, representing a quarter turn.
Select the obtuse angle:
We divide a full revolution up into $360$360 small turns called degrees, and write the unit using a small circle, like this: $360^\circ$360°.
Since $90$90 is one quarter of $360$360, we know that a right angle is exactly $90^\circ$90°. This circle has markings every $45^\circ$45°:
We can measure angles more precisely using a protractor, or an applet like this one:
This lets us associate numbers with the angle types we learned about above.
A full revolution is made up of $360$360 degrees, a single degree is written $1^\circ$1°.
|Angle type||Angle size|
|Acute angle||Larger than $0^\circ$0°, smaller than $90^\circ$90°.|
|Obtuse angle||Larger than $90^\circ$90°, smaller than $180^\circ$180°.|
|Reflex angle||Larger than $180^\circ$180°, smaller than $360^\circ$360°.|
Select the angle that is closest to $120^\circ$120°:
Use facts about supplementary, complementary, vertical, and adjacent angles in a multi-step problem to write and solve equations for an unknown angle in a figure.