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Performing sequences of transformations
Sal reflects and then translates a given pentagon, and determines whether the resulting figure is congruent to the source figure. Created by Sal Khan.
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- I wish these videos have closed captioning because i am deaf and i cant hear whats going on. all i know is pointy pointy read slide pointy. /.\(8 votes)
- You CAN use closed captioning friend! Start the video and then once it begins, immediately pause it . Move to the R lower corner of the video screen. Underneath the youtube logo there is a row of icons. The first is a cc in what looks like a tv, this is closed captioning toggle. Click on it to set up your closed captioning for all videos: it is just to the left of the icon for settings that looks like a "gear" you only need to set it up once and it will work for all future videos. Good luck!(10 votes)
- At0:52Sal said
so a reflection over the line y equals x minus 2.
What does this mean?(6 votes)- That is the equation of the line. It gives you the information from which to pick two points to reflect over. Every y value has a corresponding value of x - 2. For example, if y is 2, x is 0.(0 votes)
- how to do clockwise 90 degree's(2 votes)
- positive degrees are counter clockwise so negative are clockwise. select your dilation point and go -90 degrees.(3 votes)
- I don't get it when at1:38sal says that when x is 2 y is 0 but how does he know that x is 2??(2 votes)
- He was just trying to pick two points that were easy to find, so if he saw x - 2, giving x a value of 2 is easy. He could have picked any number.(3 votes)
- i am confused because he uses terms i dont know(3 votes)
- If you don't know the terms you can look them up.(0 votes)
- how do u get the coordianates, and angle measurement when rotating?(2 votes)
- you plug a number into the slope formula(0 votes)
- oh so you input a number into x to get y and then whatever you input back into y you will get x. Oh so thats how reflection works.(1 vote)
- This seems very confusing. It feels like I have missed something. In the reflection process, I see how Sal picked zero to represent x. I see how if x=0, then y will be -2. I'm getting lost when he then decides that x will be 2 and y will be 0. I'm guessing that he is simply pulling these numbers because this is where he wants his reflection line to go? Also, after reflecting & translating, the figure has moved to a weird overlapping position. I fail to see what this proved? We knew they were congruent at the beginning?(1 vote)
- he did not represent x by zero he plugged zero into the function y=x-2 and solved for y.
then Sal does the same with the value 2, he sets x to 2 and solves for y
y=x-2
y=2-2
y=0
part of the question was to reflect and translate, the point of the exercise is to make it clear that translation, reflection, rotation has no effect on congruence unlike resizing(dilation).(1 vote)
- How do you pick points for the reflections? It makes no sense to me.(1 vote)
- You probably figured it out in the meantime. But in case you hav'nt, at around1:18Sal says that you basically just pick two arbitrary points which are on the line of reflection.
If you want to make a reflection across the line y=3x, then you could pick (1,3) and (2,6), or any other two point satisfying the equation.(1 vote)
- At1:29how did he know what x and y equaled? And why were there two parts to what they equaled?(1 vote)
Video transcript
Starting with the figure
shown, perform the following transformations--
reflection over the line y equals x minus 2, translation
by 3 in the x direction. So we're going to
shift to the right by 3 and then shift down by 9. Is a transformed
figure congruent to the original figure? And actually, we could
answer this question before we even do
the transformation, because this transformation
only involves reflections and translations. As long as we're only
reflecting, translating, and rotating, we're going to
get to a congruent figure. The thing that will make it
not necessarily congruent is when we dilate it. So dilating it is going to
actually scale it up or down. The figure will
still be similar, but it won't necessarily
be congruent anymore. So I could already say yes,
the figures are congruent. But let's actually
perform the transformation that they want us to. So a reflection over the
line y equals x minus 2. And so let's click on translate,
and this isn't popping up-- this isn't allowing us
just to do it by hand, or do it with our mouse. They want us to think about how
much-- actually we don't want to translate first, we
want to reflect first. So once again, the reflection
line tool doesn't show up. Instead, for this
exercise, they want us to specify two
points on the line that we want to reflect around--
because two points define a line. So what are two points that are
on the line y equals x minus 2? Well when x is equal
to 0, y is negative 2. And then another
point on it that might jump out at you--
when x is 2, y is 0. So we just need any two
different points on that line. And then notice,
y equals x minus 2 is a line that looks like this. I'm just going to
kind of trace it. I don't have my drawing
tool out right now, so it's a line that looks
something like that. Actually, let me do
it, just because I think it'll be
interesting to look at. Let me copy and paste
this right over here. And then let me put it
onto my actual scratch pad. So that's from a
previous problem. So let me clear all
of this business out. So let me paste it
right over here. And notice, we just
reflected around the line y equals x minus 2.
y equals x minus 2. Let's see, it has its
y-intercept right over there, it has a slope of 1. So this is the line that
we just reflected around. And it looks pretty clear
that that is what happened. So just like that, I know
my line isn't as straight as it could be, but you
get the general idea. That is the line y is
equal to x minus 2. Notice each of these points--
you drop a perpendicular from that point,
whatever distance that is, go the same
distance on the other side, and we have reflected over. Drop a perpendicular, same
distance on the other side. And so that is our
reflection about the line y equals x minus 2. But we are not done yet. We now have to perform the
translation by 3, negative 9. So we're going to translate. So when we do it
by 3, it's going to shift this to the right by 3. And then negative 9, which is
going to shift it down by 9. And we see that it did. And we already said, these
are definitely congruent. We haven't scaled
this up or down, we've just translated it,
rotated it-- actually, we didn't even rotate it. We've just translated it, and
reflected-- or reflected it, and translated it. But let's check our answer, just
to feel good about ourselves. And we got it right.