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.
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.
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.
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
circle, we say the angle is
360 degrees (360°).
This angle is half of the full circle,
so it measures 180°. It is called
the straight angle.
This is one-fourth of the
full circle, so it is 90°.
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
angle: an angle that is
more than a right angle,
yet less than a straight
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.
| Which of these two angles is bigger? Look at how much the angle has opened? How big a part of a circle have the sides drawn? The second angle (on the right) is bigger. |
| Many times the arrows are omitted from the rays, and the arc of the circle is drawn as a tiny arc near the vertex. Even that is not necessary. Which of these is a bigger angle? Again, the second one. |
4. Which angle is bigger?
| a. | OR |
