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.
Angles are a measure of turning. All angles can be compared to a right angle, representing a quarter turn.
Select the obtuse angle:
Angles are a measure of turning. All angles can be compared to a right angle, representing a quarter turn.
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.
We can measure angles more precisely using a protractor, or an applet like this one:
The applet shows the sizes of the different kind of angles: acute, right, obtuse, straight, reflex and full revolution.
This lets us associate numbers with the angle types we learned about above.
A full revolution is made up of 360 degrees, a single degree is written as 1\degree.
Angle type | Angle size |
---|---|
\text{Acute angle} | \text{Larger than } 0 \degree, \text{ smaller than } 90\degree. |
\text{Right angle} | 90\degree |
\text{Obtuse angle} | \text{Larger than } 90\degree, \text{ smaller than } 180\degree. |
\text{Straight angle} | 180\degree |
\text{Reflex angle} | \text{Larger than } 180\degree, \text{ smaller than } 360\degree. |
\text{Full revolution} | 360\degree |
Select the angle that is closest to 120\degree:
A full revolution is made up of 360 degrees, a single degree is written 1\degree.
Angle type | Angle size |
---|---|
\text{Acute angle} | \text{Larger than } 0 \degree, \text{smaller than } 90\degree. |
\text{Right angle} | 90\degree |
\text{Obtuse angle} | \text{Larger than } 90\degree, \text{smaller than } 180\degree. |
\text{Straight angle} | 180\degree |
\text{Reflex angle} | \text{Larger than } 180\degree, \text{smaller than } 360\degree. |
\text{Full revolution} | 360\degree |
Whenever two angles share a defining line, ray, or segment, and do not overlap, we say they are adjacent angles. Here are some examples:
Whenever two segments, lines, or rays intersect at a point, two pairs of equal angles are formed. Each angle in the pair is on the opposite side of the intersection point, and they are called vertically opposite angles.
Four angles are formed by the intersecting lines, and there are two pairs of equal angles. Each pair are vertical angles.
If two angles form a right angle, we say they are complementary. We then know that they add to 90\degree.
If two angles form a straight angle, we say they are supplementary. We then know that they add to 180\degree.
Whenever we know that two (or more) angles form a right angle, a straight angle, or a full revolution, we can write an equation that expresses this relationship.
The angles in the diagram below are complementary. What is the value of x?
Adjacent angles are two angles sharing a defining line, ray, or segment, and do not overlap.
Vertical angles are two pairs of equal angles formed whenever two segments, lines, or rays intersect at a point.
If two angles form a right angle, we say they are complementary.
If two angles form a straight angle, we say they are supplementary.