Reflecting points across horizontal and vertical lines

- [Instructor] We're asked
to plot the image of point A

under a reflection across the line l.

So we have our line l here,

and so we wanna plot the image of here,

we wanna plot the image of point A

under a reflection across line l.

Well, one way to think about it is point A

is exactly one, two, three,
four units to the right of l.

And so its reflection is going to be

four units to the left of l.

So if we go one, two, three, four,

that would be the image of point A.

We could maybe denote that as A prime.

So if you're doing this on
the Khan Academy exercise,

you would actually just click
on a point right over there,

and it would show up.

But this would be the reflection

of point A across the line l.

Let's do another example.

So here we're asked plot
the image of point B

under a reflection across the x-axis.

Alright, so this is point B,

and we're going to reflect it

across the x-axis right over here.

So to go from B to the x-axis,

it's exactly five units below the x-axis.

One, two, three, four, five.

So if we were to reflect
across the x-axis,

essentially create its mirror image,

it's going to be five
units above the x-axis.

One, two, three, four, five.

So that's where the image would be.

Maybe we could denote that with a B prime.

We are reflecting across the x-axis.

Let's do another example.

So here they tell us point
C prime is the image of C,

which is at the coordinates negative four

comma negative two, under a
reflection across the y-axis.

What are the coordinates of C prime?

So pause this video

and see if you can figure
it out on your own.

So there's several ways to approach it.

It doesn't hurt to do
a quick visual diagram.

So that could be my x-axis.

This would be my y-axis.

And it's the point negative
four comma negative two,

so that might look like this.

Negative four, negative two.

So this is the point C right over here.

And we wanna reflect across the y-axis.

So we wanna reflect across the y-axis,

which I am coloring it
in red right over here.

So let's see.

The point C is four to
the left of the y-axis.

So its reflection is going to be

four to the right of the y-axis.

So let me do it like this.

So instead of being four to the left,

we wanna go four to the
right, so plus four.

So where would that put our C prime?

So our C prime would be right over there.

And what would its coordinates be?

Well, it would have the same y-coordinate,

so C prime would have a
y-coordinate of negative two.

But what would its x-coordinate be?

Well, instead of it being negative four,

it gets flipped over the y-axis,

so now it's gonna have a
x-coordinate of positive four.

So the coordinates here would
be four comma negative two.

Four comma negative two.

You might've been able
to do this in your head.

Although, for me, even if
I try to do it in my head,

I would still have this
visualization going on in my head.

Negative four comma negative two.

I'm sitting there in the third quadrant.

If I'm flipping over the y-axis,

my y-coordinate wouldn't change,

but my x-coordinate
would become the opposite

and I would end up in the fourth quadrant,

and that's exactly what happened.

Y-coordinate did not change,
but then my x-coordinate,

since I'm flipping over the y-axis,

it became the negative of this,

so the opposite of negative
four which is positive four.