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## High school geometry

### Unit 2: Lesson 3

Properties & definitions of transformations# Identifying type of transformation

CCSS.Math:

Sal is given information about a transformation in terms of a few pairs of points and their corresponding images, and he determines what kind of transformation it can be. Created by Sal Khan.

## Want to join the conversation?

- I've never been more frustrated in my life, I do not get frustrated easily, as a matter of fact I've never been frustrated, this is the first time I feel angry at math exercises, the quantitatively defining regid transformations is really bad, there is no explanation as to how you can find out where the center of rotation is, there must be a tool that helps you find where you can place the center to do a rotation affecting multiple points.(2 votes)
- Look at the hints and write them down. After a few problems, it'll all make sense.(3 votes)

- At2:58, Sal said that only reflection and rotation works for transformation C. But wait, if Sal translates the first line 9 units to the right and 6 units down, you can see that the first line maps exactly onto the second line! So wouldn't translation work too on transformation C?(4 votes)
- You are mapping point(-2,3) into point(4,-1) and point(-5,5) into point(7,-3), and not the two lines.(2 votes)

- I'm a bit confused about reflection for transformation D because it technically does have a line of reflection. (-7/2,4) for -5,5 to 2,-3 and (5.5,-2) for 4,-1 to 7,-3.(3 votes)
- You have to do the transformation from the two points(which has two groups) in one transformation.(2 votes)

- I'm really confused why transformation d cannot be a reflection nor rotation(2 votes)
- The two sets of points both slide along the same vector of <3,-2>. A rotation would generally be the same numbers (though the order and signs might be different) if you rotate about origin. A reflection also generally has the same numbers, but different order.(2 votes)

- An associated Exercise says there exists a Map which takes A to A' and B to B'. It also asserts that the quadrilateral AA'BB' is a parallelogram. The hints say thats proof the Map is a translation.

But what if the translation for A to A' is parallel but opposite in direction to the translation of B to B'?

Usually in these exercises, we get it wrong if we assume something not given. (E.g., in rotation questions when they say the angle after rotation is the same but they omit that distance is unchanged, the answer is that's not enough info to call it a rotation.)

What am I missing, please?(2 votes)- It wouldn't be a parallelogram if you translated A in one direction and B in the opposite direction. The line A' to B' would cross the line from A to B, forming two triangles rather than a parallelogram. It helps me to sketch out the situation.(2 votes)

- In1:15, why can't it be a translation? I didn't understand.(2 votes)
- Because one point would have to move 6 to the right and 4 down. While the other would move 12 to the right and 8 down. In a translation, all points should be moved the same distance.(2 votes)

- In the first question why does Sal say a rotation makes more even more sense than a reflection ?2:43(2 votes)
- Probably because all the points are on the same line, so it is quite clear that there is a 180 degree rotation.(2 votes)

- what does it mean by

Preserves angle measures and segment lengths

Preserves all angle measures only

Preserves all segment lengths only?(2 votes)*Preserves angle measures and segment lengths*: means that after whatever transformation you perform, the angles are the same and the lengths of the sides are also unchanged. For instance, if you have a triangle and you translate it by (-7, 3) it is still exactly the same size with the same angles. Ditto for rotations.*Preserves all angle measures only*: means that the lengths may change, but the angles remain the same. For instance, in a dilation, the figure gets bigger or smaller, but the angles don't change.

(BTW, for those that have done the congruent/similar playlists, the first is describing a figure that is congruent to the original figure after transformation and the second is describing a figure that is no longer congruent but is still similar to the original figure.)*Preserves all segment lengths only*: The angles change but the sides remain the same length. Does anyone have an example for this one?(2 votes)

- Out of curiosity, I wonder where the exercises for this lesson have gone. (At least, I don't see them in this section.) Anyone know?(2 votes)
- I think it's just an extra video to help you understand more.(2 votes)

- This video makes sense, I get all parts shown is this video and the past two on the subject. However they are nothing like the questions I have been getting in Precisely defining rigid transformations exercise. Also some of those questions don't even have points to the point where i'm not able to figure it out. (I'm talking about the questions formatted with the bubbles, no graphs what so ever). So what do I do about this exercise when I can't even do it.(2 votes)
- Open all of the hints and read them and take notes, then use that information when you go through more questions.(2 votes)

## Video transcript

Transformation C maps negative
2, 3 to 4, negative 1. So let me do negative
2 comma 3, and it maps that to 4, negative 1. And point negative 5 comma 5,
it maps that to 7, negative 3. And so let's think
about this a little bit. How could we get from
this point to this point, and that point to that point? Now it's tempting to view this
that maybe a translation is possible. Because if you imagined a
line like that, you could say, hey, let's just shift this
whole thing down and then to the right. These two things happen
to have the same slope. They both have a
slope of negative 2/3, and so this point would
map to this point, and that point would
map to that point. But that's not what we want. We don't want negative 2,
3 to map to 7, negative 3. We want negative 2, 3
to map to 4, negative 1. So you could get this
line over this line, but we won't map the
points that we want to map. So this can't be, at least
I can't think of a way, that this could actually
be a translation. Now let's think about
whether our transformation could be a reflection. Well, if we imagine a
line that has-- let's see, these both have a
slope of negative 3. These both have a
slope of negative 2/3. So if you imagined a line that
had a slope of positive 3/2 that was equidistant from both--
and I don't know if this is. Let's see, is this equidistant? Is this equidistant
from both of them? It's either going to be that
line or this line right over-- or that line, actually
that line looks better. So that one. And once again, I'm
just eyeballing it. So a line that has
slope of positive 3/2. So this one looks right
in between the two. Or actually it could be
someplace in between. But either way, we just have to
think about it qualitatively. If you had a line that
looked something like that, and if you were to
reflect over this line, then this point would
map to this point, which is what we want. And this purple point,
negative 5 comma 5, would map to that point. It would be reflected over. So it's pretty clear that
this could be a reflection. Now rotation actually
makes even more sense, or at least in my brain
makes a little more sense. If you were to rotate around
to this point right over here, this point would
map to that point, and that point would
map to that point. So a rotation also
seems like a possibility for transformation C. Now let's think
about transformation D. We are going from 4,
negative 1 to 7, negative 3. Actually maybe I'll put
that in magenta, as well. To 7, negative 3,
just like that. And we want to go from
negative 5, 5 to negative 2, 3. So I could definitely imagine
a translation right over here. This point went 3 to
the right and 2 down. This point went 3 to
the right and 2 down. So a translation
definitely makes sense. Now let's think
about a reflection. So it would be
tempting to-- let's see, if I were to get from
this point to this point, I could reflect around that, but
that won't help this one over here. And to get from that
point to that point, I could reflect around
that, but once again, that's not going to help
that point over there. So a reflection really
doesn't seem to do the trick. And what about a rotation? Well to go from this
point to this point, we could rotate
around this point. We could go there, but that
won't help this point right over here. While this is rotating
there, this point is going to rotate
around like that and it's going to end
up someplace out here. So that's not going to help. So it looks like this one
can only be a translation.