1. Triangle Geometry Grade 5
Objectives:
After completing this activity you will be able to identify triangles by their angles and their side lengths.
After completing this activity you will be able to identify two triangles as congruent.
Are you ready? Click on the house to continue to the main menu. This button will return you to the main menu at any point. Let’s go!
Created by Ruth Myles
CEP 811

2. Menu Identifying triangles by angle size
Identifying triangles by length
Identifying triangles as congruent
Practice

3. Angle sizes Stop and Think! What do you notice about these angles?
Continue

4. Acute Angle- Less than 90 degrees
Did you notice that these angles are all small? We call angles that are less than 90 degrees acute. You can remember this by thinking of “a cute” little angle.
Notice how the 90 degree angle looks like a chair? This angle is called a right angle. Any angle that is smaller than that chair is our acute little angle! .
Right Angle- 90 degrees
Acute Angle- Less than 90 degrees
Remember:
90 degree angles are chairs, they are “right” to sit on.
Acute angles are less than 90 degrees, they are “ a cute” little angle

5. Stop and Think! Do you know what we call the last angle? Right Angle
Acute Angle
????

6. Obtuse Angles
Any angle larger than 90 degrees is called obtuse. Can you think of a way to remember that the obtuse angle is a big angle?
Right Angle- 90 degrees
Obtuse angle- More than 90 degrees

7. Let’s look at some examples!
(Click to check your work)
Can you figure out which 2 arrows are
pointing to obtuse angles?

8. Let’s look at some examples!
Can you figure out which 2 arrows are
pointing to acute angles?

9. Let’s look at some examples!
Can you figure out which 3 arrows are
pointing to right angles?

10. Triangles
Angle sizes help us to identify triangles too! Look at the triangles below, and see if you can guess what kind of triangle they are: Click the triangles to check your work

11. Great Job! Use the home button to choose a new topic or to start the practice session.

12. Triangle Side Length
You already know that we can name triangles by their angles…. Did you know we can name them by their side length too? Look at this triangle and think about what’s special about it.

13. Maybe you noticed…
It’s blue
It’s an acute triangle
The angles are all the same size
What about the sides of the triangle? When all the sides of a triangle are the same or equal we call it an equilateral triangle!

14. What if all the sides are not equal?
This triangle has two equal sides. It is called an isosceles triangle. Some students think they look like icicles.
This triangle has no sides that are the same length. It is called a scalene triangle. Can you think of a way to remember that scalene triangles have no equal lengths?

15. Review
Can you tell what kind of triangles these are by looking at the lengths of their sides? Click and listen to check your work. Use the home button to choose a new topic or to start the practice session.

16. What does congruent mean?
Triangles are congruent if they are the same shape and size. Are these triangles congruent?

17. We can tell if two triangles are congruent by placing them on top of each other. If they match, they are congruent.
These triangles are the same shape, but they are not the same size. They are not congruent.

18. How else can we tell if triangles are congruent?
We can’t always place two shapes on top of each other. Can you think of another way of telling if these triangles are congruent?

19. Measure it!
1.5 inches
Looking at these measurements, we can tell that these triangles are congruent! They are the same shape, and the same size.
1.5 inches
2.5 inches
2 inches
2.5 inches
2 in.

20. Now it’s time to practice!
On the following pages answer the questions the best you can. If you need to review a section, click on the home button.

21. What kind of triangle is this?
Obtuse
Acute
Equilateral

22. What kind of triangle is this?
Right
Acute
Obtuse

23. What type of triangle is this?
Scalene
Obtuse
Equilateral

24. What type of triangle is this?
Isosceles
Scalene
Right

25. What type of triangle is this?
Equilateral
Acute
Obtuse

26. What type of triangle is this?
Equilateral
Scalene
Isosceles

27. What makes triangles congruent?
The angles in both triangles are the same.
The shape of the triangle is the same.
The shape and size are the same.

28. Are these triangles congruent? Why?
Yes, because they are the same shape.
No, because they are different colors.
No, they are different sizes.

29. Are these triangles congruent? Why?
No, they are different colors.
No, they are facing different directions.
Yes, they are the same shape and size.
1 inch
1 inch
1 inch
1 inch
1 inch
1 inch

30. Can you find the acute equilateral triangle?

31. Can you find the right scalene triangle?

32. Can you find the obtuse isosceles triangle?

33. Can you find the acute isosceles triangles?

34. Great Job!