Created by Ruth MylesCEP 811

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!

MenuIdentifying triangles by angle size

Identifying triangles by length

Identifying triangles as congruent

Practice

Angle sizesStop and Think!

What do you notice about these angles?

Continue

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!

.

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

Right Angle- 90 degrees Acute Angle- Less than 90 degrees

Stop and Think!Do you know what we call the last angle?

Right Angle Acute Angle ????

Obtuse AnglesAny 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

Let’s look at some examples!(Click to check your work)

Can you figure out which 2 arrows arepointing to obtuse angles?

Let’s look at some examples!Let’s look at some examples!

Can you figure out which 2 arrows are pointing to acute angles?

Let’s look at some examples!

Can you figure out which 3 arrows are pointing to right angles?

TrianglesAngle 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

Triangle Side LengthYou 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.

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!

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?

ReviewCan 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.

What does congruent mean?

Triangles are congruent if they are the same shape and size. Are these triangles congruent?

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.

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?

Measure it!

Looking at these measurements, we can tell that these triangles are congruent! They are the same shape, and the same size.

2 in.

2.5 inches

1.5

inch

es

1.5 inches

2.5

inch

es

2 in

ches

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.

What kind of triangle is this?

A.Obtuse

B.Acute

C.Equilateral

What kind of triangle is this?

A.Right

B.Acute

C.Obtuse

What type of triangle is this?A. Scalene

B. Obtuse

C. Equilateral

What type of triangle is this?A.Isosceles

B.Scalene

C.Right

What type of triangle is this?

A.Equilateral

B.Acute

C.Obtuse

What type of triangle is this?A.Equilateral

B.Scalene

C.Isosceles

What makes triangles congruent?A. The angles in both triangles are the

same.

B. The shape of the triangle is the same.

C. The shape and size are the same.

Are these triangles congruent?Why?

A. Yes, because they are the same shape.

B. No, because they are different colors.

C. No, they are different sizes.

Are these triangles congruent? Why?

A. No, they are different colors.

B. No, they are facing different directions.

C. Yes, they are the same shape and size.

1 in

ch

1 inch

1 inch

1 inc

h

1 inch

1 inch

Can you find the

acute equilateral triangle?

Can you find the

right scalene

triangle?

Can you find the obtuse isosceles triangle?

Can you find the

acute isosceles triangles?