Video Transcript
Lines of Symmetry
In this video, we’re going to learn
how to draw lines of symmetry in shapes and figures and count all the lines of
symmetry in given shapes. If we look around us, we can see
lots of examples of symmetry in real life. This leaf is symmetrical and this
is where we would draw the line of symmetry. If we were to fold the leaf in half
along the line of symmetry, it would look like this and the other half would look
like this. And if we flip this leaf over like
so, we could place one-half of the leaf on top of the other and they would be a
complete match. This is how we know the leaf is
symmetrical.
Where would you draw the line of
symmetry on this picture of a crab? You should go here. Sometimes people call the line of
symmetry a mirror line because each half is a mirror image of the other half. If we were to place a mirror on
this line of symmetry, the reflection would look like this. This is another way to tell if a
shape is symmetrical. Some shapes are symmetrical like
this rectangle, and this is its line of symmetry. If we rotate or turn this
rectangle, we can see another line of symmetry. Some shapes have more than one line
of symmetry. This rectangle has two.
How many lines of symmetry does
this triangle have? We’ve drawn one line of symmetry
from the vertex at the top of the triangle down through the middle of the shape. And if we turn or rotate the
triangle, we could draw another line of symmetry from the vertex at the top of the
triangle through the middle of the shape like so. So far, we found two lines of
symmetry. If we rotate the triangle again, we
could draw another line of symmetry. Starting at the vertex at the top,
again, we can draw a line all the way through the middle of the triangle, so this
equilateral triangle has three lines of symmetry.
So far, we’ve learned how to draw
lines of symmetry onto shapes or figures and how to count the number of lines of
symmetry a shape or figure has. Let’s try some questions now and
put into practice what we’ve learned.
Which of these are lines of
symmetry of the letter? Line A only, lines A and C, line B
only, lines A and B, or line C only.
In this question, we’re shown a
capital letter A and it has three lines drawn on it, lines A, B, and C. We have to decide which of these
lines are lines of symmetry of the letter A. Let’s start by looking at line
A. Is this line a line of
symmetry? One way we could find out is to
think about what would happen if we folded this letter along line A. One-half would look like this, and
the other half would look like this. We couldn’t fit each half on top of
each other and make a match. So line A is not a line of
symmetry. So we can eliminate line A as a
possible answer and any of the possible answers which have line A in them.
What about line B? Is this a line of symmetry? If we were to fold along the line
B, each half would look like this. We can’t fit one-half on top of the
other. And if we were to place a mirror
line along line B, we end up with a very funny-looking letter. Line B is not a line of symmetry
either. So line C must be the one which is
a line of symmetry. Let’s check. If we fold along the line of
symmetry, one-half of the letter A would look like this and the other half would
look like this. If we were to flip this half over
like so, we could fit it perfectly on top of the other half like so. This is how we know that line C is
a line of symmetry. So the correct answer is line C
only. This is the only one of the three
lines which is a line of symmetry of the capital letter A.
How many lines of symmetry does
this figure have?
In this question, we’re shown this
star figure, and we’re asked to work out how many lines of symmetry it has. We could start from this vertex
here at the top of the star and draw a line of symmetry cutting through the middle
of the star like so. If we were to place a mirror along
this line of symmetry, the reflection would look like this. This is how we know this shape is
symmetrical and the line is a line of symmetry. So far, we found one line of
symmetry in the star shape. If we turn or rotate the star so
that one of the points is pointing upwards, we could draw another line of symmetry
going through the middle of the shape, just like before.
So far, we’ve found two lines of
symmetry. In the same way, we can rotate or
turn the star and draw another line of symmetry. We’ve found three lines of
symmetry. By turning the shape again, we can
draw a fourth line of symmetry. And if we turn the shape one more
time, we can draw another line of symmetry. Altogether, we counted five lines
of symmetry. By starting at the top point each
time and drawing a line down through the middle of the shape, we were able to draw
five lines of symmetry. This star shape has five lines of
symmetry.
What have we learned in this
video? We’ve learned how to draw lines of
symmetry in shapes or figures. We also learned how to count the
lines of symmetry in given shapes or figures.
