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9.02 Measuring angles

Lesson

Angle types

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 segments perpendicular to each other forming right angle.

Two perpendicular segments.

We draw all other angles with a circular arc.

An acute angle

An angle that is smaller than a right angle is called an acute angle.

Two rays forming a straight angle.

Two right angles together form a straight angle.

A full revolution

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

An obtuse angle

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

A reflex angle

We met this kind of angle in the previous lesson - a reflex angle which 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.

A circle with markings at 0, 90 degrees and 180 degrees. Ask your teacher for more information.

Examples

Example 1

Select the obtuse angle:

A
Two rays forming a straight angle
B
Two segments forming a right angle
C
An obtuse angle
D
An acute angle
Worked Solution
Create a strategy

Use this image to help you:

A circle with markings at 0, 90 degrees and 180 degrees. Ask your teacher for more information.

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

Apply the idea

The answer is Option C.

An obtuse angle
Idea summary

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

A circle with markings at 0, 90 degrees and 180 degrees. Ask your teacher for more information.

Measure angles

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.

A circle showing the angles at every 45 degrees from 0 to 360.

This circle has markings every 45\degree.

Exploration

We can measure angles more precisely using a protractor, or an applet like this one:

Loading interactive...

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 typeAngle 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

Examples

Example 2

Select the angle that is closest to 120\degree:

A
A right angle.
B
An obtuse angle.
C
An obtuse angle
D
An obtuse angle
Worked Solution
Create a strategy

Use this image to help you:

A circle with markings every 45 degrees.

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

Apply the idea

A 120\degree angle would be an obtuse angle between 90\degree and 135\degree.

The answer is Option C.

An obtuse angle
Idea summary

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

Angle typeAngle 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

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:

Two sets of adjacent angles. Ask your teacher for more information.

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.

3 images. Each showing vertically opposite angles formed by 2 intersecting lines. Ask your teacher for more information.

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.

An image showing 2 complementary angles forming a right angle and 2 supplementary angles forming a straight angle.

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.

Examples

Example 3

The angles in the diagram below are complementary. What is the value of x?

A right angle formed by complementary angles of 39 degrees and x degrees.
Worked Solution
Create a strategy

Complementary angles are two angles forming a right angle equivalent to 90\degree.

Apply the idea
\displaystyle x + 39\displaystyle =\displaystyle 90Equate the sum of the angles to 90
\displaystyle x\displaystyle =\displaystyle 51Subtract 39 from both sides
Reflect and check

We never use degrees once we are working with an equation. We are solving for the value of x, and we don't want to double up on using the degree symbol.

Idea summary

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.

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