Lesson video

Hi there, welcome to this lesson.

For this lesson we will be looking at two types of triangles and they are scalene triangles and isosceles triangles, but before we start, my name is Miss Darwish, and what I'd like you to do is just take yourself to a nice, quiet place so we can begin the lesson.

Hi, there, so for today's lesson we are going to be comparing and classifying isosceles and scalene triangles.

So the agenda for today is as follows.

We are going to be recapping triangles and then we're going to be looking at isosceles triangles followed by scalene triangles.

And then at the end of course there will be a quiz for you to do.

So you will need, for today, just a pencil, something to write with, and something to write on.

So if you want to go and get those items and we can start.

Okay, properties of triangles.

What do you know about triangles? Quickly have a think.

I'm going to give you six seconds to have a think.

What do you know about triangles? Okay, triangles have three straight sides of course and triangles have three vertices.

Well done if you said that.

Now, what about the angles of triangles? What do you know about the angles of a triangle? The three angles in a triangle of course add up to 180 degrees.

Well done if you said that.

Okay, here is a triangle.

What do you notice about this triangle? What do you notice about the angles? Each angle is 60 degrees, and because all three angles are 60 degrees, this triangle is called an equilateral triangle.

And if we were to measure the sides, they would all be equal.

And if we were to measure with a protractor the angles, they are all equal.

And because the sides and the angles are equal to each other this is an equilateral triangle.

Okay, here is a different triangle.

What do you notice? Can you see that two of the angles have got the same measurement? That's 70 degrees and 70 degrees, and the third angle has a measurement of 40 degrees.

This is not an equilateral triangle.

Tell me, why is it not an equilateral triangle? Tell me.

Okay, good, because all three angles are not the same.

An equilateral triangle has to have each angle in an equilateral triangle has to be equal to 60 degrees.

This is definitely not an equilateral triangle, but two of the angles are actually the same, so we would call this triangle an isosceles triangle, why? Because two of the angles are the same.

Why? tell me again.

Because two of the angles are the same.

And because two of the angles are the same, if we were to measure with a ruler, two of the sides would also be the same.

Okay, here is an example of another isosceles triangle.

So there's a 50 degrees, 65 degrees, and 65 degrees.

Why is this an isosceles triangle? Good, this is an isosceles triangle because two of the angles are equal to each other.

Okay, now, my tip to remember an isosceles triangle is because it sounds like isosceles, I sausages, and usually maybe you'd have on your plate of food two sausages, so two of the measurements are the same.

Two of the angles are the same.

Just like you would have two sausages on your plate.

And also, with my class before, we used to make isosceles dogs and you'd remember the isosceles dog, 'cause you can see from this picture, because the ears are where the two angles that are the same are, okay? So isosceles dog, isosceles as in I sausages, that's how I remember that only two of the angles or two of the measurements are equal.

Okay, here is another isosceles triangle, or is it not an isosceles triangle? We need to have a think.

Tell me, is it an isosceles triangle, or is it not an isosceles triangle, and why? Explain.

What do you notice about the angles? Okay.

We only know two out of three of the angles, 80 degrees and 50 degrees.

What would the third one be? So let's find the missing angle and then we're going to decide, is this an isosceles triangle, or is it not an isosceles triangle? So 50 degrees add 80 degrees is equal to 130.

The angles inside a triangle add up to 180, so that's 50 away.

That means the next angle is 50 degrees, so the missing angle is 50 degrees.

So we've got 50 degrees, 50 degrees, and 80 degrees, and that makes 180 degrees.

So is this an isosceles triangle? Yes, it is, because two of the angles are the same, 50 degrees and 50 degrees.

Okay, hmm, is this an isosceles triangle? What do you think? If it is, why? If it's not, why not? I'll give you a clue, find the missing angle first.

That's always your next step.

I don't know if it's an isosceles triangle quite yet, because I've only got two out of three of the angles.

So let's find the third angle and then we'll decide.

Okay, so we've got 90 degrees, 45 degrees, what's the missing angle? So 90 add 45 is equal to 135.

And if we take that away from 180, it is equal to, if we do 180 take away 135, it is equal to 45 degrees.

So 45 degrees is the missing angle.

Are two of the angles the same? They are the same, they are both 45 degrees.

That means this is an isosceles triangle.

Well done if you said that.

But, did you also notice that it's also a right angled triangle? Why is it a right angled triangle? Because one of the angles is 90 degrees.

So is it possible for a triangle to be 90 degrees and an isosceles triangle? It is definitely possible, and this is the example.

One of its angles are 90 degrees and the other two angles are the same, so it's a 90 degree triangle, or a right angled triangle, and it is an isosceles triangle.

Okay, now, here's another triangle.

What do you notice about this triangle? What do you notice about this triangle? All the angles have got different measurements.

60 degrees, 70 degrees, 50 degrees.

So all the angles have different measurements.

Is it an isosceles triangle? Tell me, why is it not an isosceles triangle? Because an isosceles triangle has to have two angles with the same measurement.

This does not have that.

Is this an equilateral triangle? Because an equilateral triangle has to have three angles which are all equal to 60 degrees.

Is this a right angled triangle? No, because there is no right angle, there is no 90 degree angle, so what is this triangle? It is a scalene triangle.

Can you say the word scalene for me? This is a scalene triangle.

So a scalene triangle is a triangle where all three angles have got different measurements.

So if I was to measure all the sides with a ruler, they would all be completely different.

And if I was to measure with my protractor the angles, they would all be completely different, and that is why it would be a scalene triangle.

Okay.

Hmm, let's have a look.

Is this a scalene triangle? How do we know? Because all three angles are different.

What else do we notice about this triangle? Did you notice that right angle? So this is a scalene triangle and a right angled triangle.

So before, we saw an example where we had a triangle that had 90 degrees, it was a right angled triangle, and it was an isosceles triangle.

And now we can see an example of a triangle which is a right angled triangle and it's a scalene triangle.

Okay, I'm going to ask you just to pause the video in a few seconds and have a look at completing the tasks.

So you've just got to write the name of each of the triangles.

Is it right angled, scalene, isosceles, equilateral? And then I want you to find the third missing angle.

Remember, what do the angles in a triangle add up to? 180 degrees.

Good luck, and then come back and we will have a look and mark the answers together.

Okay, hopefully you didn't find those too difficult.

Let's have a look at the answers together.

So, this triangle, hopefully you were able to identify, if two of the angles are 60 degrees, the third one can only be 60 degrees.

So it is definitely an equilateral triangle.

So the missing angle is 60 degrees.

Well done if you got that right.

Okay, this one, you could have said it is a right angled triangle, bonus points though if said right angled triangle and a scalene triangle, why? Because it has a right angle, 90 degree angle, and the missing angle, if you did 180 take away 90 take away 35, should have left you with the missing angle, which is 55 degrees.

So all three angles are different, which is why it's a scalene triangle.

But also, one of the angles is 90 degrees, which is why also we can call it a right angled triangle.

So a very well done if you said both of these names.

Okay, the next one, we could have right angled triangle and an isosceles triangle.

It's an isosceles triangle because two of the angles are 45 degrees, as we can see, and if we take these away from 180, the missing angle is 90 degrees, which makes it a right angled triangle as well.

So bonus points and extra, extra well done if you also said right angled triangle and an isosceles triangle.

And then the last one of course is a scalene triangle.

So the missing angle is 45 degrees.

So if we did 180 degrees take away 70 degrees take away 65 degrees, that would leave us with the third missing angle, which is 45 degrees.

We would really like it if you could share your work with Oak National.

If you'd like to, then please ask your parent or carer to share your work on Twitter.

And ask them to remember to tag @OakNational and to use the #LearnwithOak.

I really look forward to seeing the work that you have done.