Lesson video

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Hi there, my name is Miss Darwish and for today's math lesson, we're going to be looking at lines of symmetry in 2D shapes.

So, before we get started, if you can take yourself to a nice, quiet environment ready to start the lesson.

Okay, so the schedule for today's lesson we're going to start off by looking at lines of symmetry in 2D shapes.

And then we're going to have a look at regular shapes.

And then we're going to just address any misconceptions that some people might have on lines of symmetry when it comes to shapes.

And of course at the end, there is a quiz for you to do.

So, you're just going to need pencil, a piece of paper and you might need a ruler as well.

Okay, so in front of me there is a triangle, as we can see.

Now, you can draw a line through a triangle as you can see, and it's considered a line of symmetry when one part is exactly the same as the other part.

So, can you see where that line is? On each side of the line, each part of the triangle is identical to the other part of the triangle.

They are the same size, they are the same shape.

So you can see from the arrows here.

So a line of symmetry is when you can draw a line through so one part is exactly the same as the other part.

Okay, now.

Different shapes have different lines of symmetry.

This shape, for example, how many lines of symmetry does it have? Just one.

So there's only one possible line I could draw through that shape where one part is the same as the other part.

So this shape only has one line of symmetry.

Some shapes have more than one line of symmetry.

So here is an example of a quadrilateral and let's count together and see how many lines of symmetry it has.

So we can see one line of symmetry.

That's a second line of symmetry.

Do you think there's any more? Or do you think it only has two lines of symmetry? Do you think there's any more, or do you think it has more than two lines of symmetry? Let's see, oh, that's a third line of symmetry.

Do you think it's going to stop up to there or do you think there's one more line of symmetry? And that's the fourth line of symmetry.

So, this quadrilateral, regular quadrilateral or square as you might want to call it, has four lines of symmetry in total.

But the shape we just saw before the square only had one line of symmetry.

So some 2D shapes have one line of symmetry, they can have zero lines of symmetry, they can have three, four.

Let's look at some more examples.

Tell me, how do we know it's a quadrilateral? Because it has four straight sides.

Now, this quadrilateral, it doesn't have any lines of symmetry.

I can't think of a line that I can draw through it where one side would be the same as the other side.

Can you? So this quadrilateral has zero lines of symmetry.

Oh, what about this heart? How many lines of symmetry can you see? Point to the screen if you think there are any lines of symmetry, or show me on your fingers, zero, one, two, three? Five, 10? 30, want to show me? Well done if you said one line of symmetry, just going straight through.

Can you see that one side of the line is exactly the same as the other side of the line? But there is isn't, no, there isn't any other option is there? So it just has one line of symmetry? Okay, this is an irregular pentagon.

How do we know it's a pentagon? Because it's a shape with five straight sides, well done.

And why is it irregular? Because the sides are not the same length.

Well done if you said that.

Okay, how many lines of symmetry do you think this irregular pentagon has? Have a think.

Okay, and show me on your fingers, how many lines of symmetry do you think it has? In three, two, one, show me.

Just one line of symmetry.

Well done if you said one line of symmetry.

Again, can you see where that line is? On each side of the shape they are identical.

Okay, my friend says that a 2D shape can only have a line of symmetry if it's a regular shape.

So my friend is saying that a 2D shape only has a line of symmetry if it's regular.

So irregular shapes do not have lines of symmetry.

That's what he's saying.

Why is he wrong? Can you think of examples to show him why he'd be wrong? Hm, okay.

A rectangle for example.

Is a rectangle a regular shape? Why? All the sides of the rectangle are not the same.

The length is a different size to the width.

You can see that on the shape, the length is a lot longer.

A rectangle is definitely not a regular shape.

But it has two lines of symmetry.

So my friend is definitely wrong, and hopefully we persuaded him with that example.

But can you think of any more examples? Hm, what about an isosceles triangle? So remind me, what's an isosceles triangle? When a triangle or a three-sided shape, where two of the angles measure the same and two of the sides measure the same.

So how many lines of symmetry do you think an isosceles triangle has? It has one line of symmetry.

And is an isosceles triangle a regular shape? Definitely not.

If it was a regular shape, a regular triangle, what's the word we use for a regular triangle? When all three sides are the same? An equilateral triangle, well done.

So my friend is definitely wrong and we've shown him two examples of why he is wrong.

Hopefully we've convinced him.

Okay, now it's time for you to pause the video and work through the independent task.

And once you've finished with that, come back and we'll go through the answers together.

Okay, hopefully they were okay for you.

We can have a look at the answers together.

Before we look at the answers together I just want to double check on something.

That if and when you came to draw your line of symmetry, you used a ruler so they're nice and straight.

Okay, so, you were asked, let me just move myself out of the way there.

Okay, to group the shapes, so you had five shapes in front of you and you had to say, did they have zero lines of symmetry, one line of symmetry or more than one line of symmetry? So we're sort of grouping them, or putting them into three categories.

So the first category, zero lines of symmetry.

Did you find any shapes with zero lines of symmetry? I did not find any shapes with zero lines of symmetry, actually the shapes that I showed you had at least one line of symmetry.

But what you could do for this section is with your ruler of course, draw a shape with any number of sides, your own shape which has zero lines of symmetry.

Okay, and then we have the group with one line of symmetry in which these three shapes only have one line of symmetry.

Hopefully you got those right.

And then the last category had the last two shapes in, and both shapes have two lines of symmetry.

So they fall under the more than one line of symmetry category.

So like I said, hopefully you got those right.

For the zero lines of symmetry, if you can draw your own shape which has zero lines of symmetry that would be great.

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