Measuring Angles And Segments Help With Geometry Homework

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Lines, Rays, and Angles

This fourth grade geometry lesson teaches the definitions for a line, ray, angle, acute angle, right angle, and obtuse angle. We also study how the size of the angle is ONLY determined by how much it has "opened" as compared to the whole circle. The lesson contains many varied exercises for students.



This is point A.
Points are named
with capital letters.
When two points are connected with a straight
line, we get a line segment. We call this line
segment AB or line segment AB(note the bar on top).
The sides of a triangle
are line segments.

A line has no beginning point or end point. Imagine it continuing indefinitely in both directions.
We can illustrate that by  little arrows on both ends.


We can name a line using two points on it. This is line EF or line (note the arrowheads).
Or, we can name a line using a lowercase letter: this is line s.

A ray starts out at a point and continues off to infinity. We can show
that by drawing an arrow at one end of the ray. Think of the sun's rays:
they start at the sun and go on indefinitely.

We can name a ray using its starting point and one other point that is
on the ray: this is ray QP or ray (note the one arrowhead). Or, we can
name a ray using a lowercase letter: this is ray r.


What is an angle? Many people think that an angle is some kind of
slanted line. But in geometry an angle is made up of two rays that
have the same beginning point

That point is called the vertex and the two rays are called the sides
of the angle.

To name an angle, we use three points, listing the vertex in the middle.
This is angle DEF or ∠DEF. We can use the symbol ∠ for angle.


1. Write if each figure is a line, ray, line segment, or an angle, and name it.


a.  _______________________

b.  _______________________


c.  _______________________

d.  _______________________

e.  _______________________

f.  _______________________

2. a. Find the angle formed by the rays DE and DF.
        How do we name it?

    b. Find the angle formed by the rays CA and CE.
        How do we name it?

    c. What is BD? (a line, a line segment, or a ray)?


3. a. Draw two points, D and E. Then draw line DE.

    b. Draw point Q not on the line.

    c. Draw rays DQ and EQ.

    d. Find angles EDQ and DEQ in your drawing.

     Imagine that the two sides of the angle start side by side, and then
open up to a certain point. When the two sides “open up”, they draw
an imaginary arc of a circle. (You can illustrate this with two pencils as the
two sides of an angle. Keep one pencil stationary while you rotate the other.)

If the angle opens up to a full
, we say the angle is
360 degrees


This angle is half of the full circle,
so it measures 180°. It is called
the straight angle.

Your two pencils (rays) are lying
down flat or straight on the floor.


This is one-fourth of the
full circle, so it is 90°.

This is called the right
Table and book
corners are right angles.


In each of these pictures the angle is opened more and more and keeps getting bigger. The arc of the circle is larger.

These angles are acute angles, which means they are less than a right angle (less than 90°). Think of acute angles as sharp angles. If someone stabbed you with the vertex of an acute angle, it would feel sharp.


The angle is opened even
more now. It is an obtuse
: an angle that is
more than a right angle,
yet less than a straight

Think of obtuse angles as
dull angles.


Here's another way of thinking about angles. Think of a sun rising in the morning in the horizon, gradually getting higher, and traveling through the sky along an arc of a circle.

How big is the angle?

It does not matter how long the sides of the angle are. Remember, they are rays, and rays go on indefinitely. But when we draw them on paper, we have to draw them as ending somewhere. 

The sides of the angle might even seem to have different lengths. That doesn't matter either. The size of the angle is ONLY determined by how much it has “opened” as compared to the whole circle. Think how big an arc of a circle the sides have drawn, as compared to a whole circle. 

4. Which angle is bigger?

5. a. Sketch three different
        acute angles.

    b. Sketch three different
        obtuse angles.

    c. Sketch a right angle
        and a straight angle.

6. Label the angles as acute, right, obtuse, or straight. To help, make these angles with two pencils,
    checking how much you need to open up the angle.


7. A triangle has three angles. In fact, the word tri-angle means a three-angled shape.

    Which of the triangles 
    a, b, or c has one
    obtuse angle?

    Which has one right angle?

a. b. c. 


8. (Optional) Make a geometry notebook where you write down each new term and draw a picture or
    pictures that illustrate the term. Use colors and tidy writing. It is like your personal geometry
    dictionary. You can also do any drawing problems from the lessons in it. Drawing and writing
    yourself, instead of just reading, can help you remember the terms better!

New Terms
  • a line
  • a line segment
  • a ray
  • an angle
  • an acute angle
  • a right angle
  • an obtuse angle
  • a straight angle

This lesson is taken from Maria Miller's book Math Mammoth Geometry 1, and posted at with permission from the author. Copyright © Maria Miller.

A line that has one defined endpoint is called a ray and extends endlessly in one direction. A ray is named after the endpoint and another point on the ray e.g.

$$\overset{\rightarrow }{AB}$$

The angle that is formed between two rays with the same endpoint is measured in degrees. The point is called the vertex

The vertex is written as

$$\measuredangle CAB$$

In algebra we used the coordinate plane to graph and solve equations.  You can plot lines, line segments, rays and angles in a coordinate plane.

In the coordinate plane above we have two rays

$$\overset{\rightarrow }{BA}\: \: and\: \: \overset{\rightarrow }{BD}$$

That form an angle with the vertex in point B.

You can use the coordinate plane to measure the length of a line segment. Point B is at (-2, -2) and C (1. -2). The distance between the two points is 1 - (-2) = 3 units.

Angles can be either straight, right, acute or obtuse.

An angle is a fraction of a circle where the whole circle is 360°. A straight angle is the same as half the circle and is 180° whereas a right angle is a quarter of a circle and is 90°.

You measure the size of an angle with a protractor.

Two angles with the same measure are called congruent angles. Congruent angles are denoted as

$$\angle A\cong \angle B$$

Or could be shown by an arc on the figure to indicate which angles that are congruent.

Two angles whose measures together are 180° are called supplementary e.g. two right angles are supplementary since 90° + 90° = 180°.

Two angles whose measures together are 90° are called complementary.

$$m\angle A+m\angle B=180^{\circ}$$

$$m\angle C+m\angle D=90^{\circ}$$

Video lesson

Measure the size of the angle

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