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:
Two perpendicular segments.
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.
Practice question
Question 1
Select the obtuse angle:
A
B
C
D
Measuring angles
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°. |
| Right angle | $90^\circ$90° |
| Obtuse angle | Larger than $90^\circ$90°, smaller than $180^\circ$180°. |
| Straight angle | $180^\circ$180° |
| Reflex angle | Larger than $180^\circ$180°, smaller than $360^\circ$360°. |
| Full revolution | $360^\circ$360° |
Practice question
Question 2
Select the angle that is closest to $120^\circ$120°:
A
B
C
D
Angle relationships
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 opposite angles.
Four angles are formed by the intersecting lines, and there are two pairs of equal angles. Each pair are opposite angles.
If two angles form a right angle, we say they are complementary. We then know that they add to $90^\circ$90°.
If two angles form a straight angle, we say they are supplementary. We then know that they add to $180^\circ$180°.
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.
Worked Example
Solve for the value of $x$x in the diagram below:
Do: We write the equation:
$x+147+116=360$x+147+116=360
We then use subtraction to make $x$x the subject:
$x=360-147-116$x=360−147−116
We then do the subtraction to find $x$x:
$x=97$x=97
Reflect: We never use degrees once we are working with an equation. We are solving for the value of $x$x, and we don't want to double up on using the degree symbol!
Practice question
Question 3
The angles in the diagram below are complementary. What is the value of $x$x?
A right angle, marked by a green box, is divided by a line segment, creating two angles: a top angle and a bottom angle. The top angle, shaded in red, measures $x$x degrees, indicating its unknown measure. The bottom angle, shaded in blue, measures $39^\circ$39°.