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## 8th grade (Illustrative Mathematics)

### Course: 8th grade (Illustrative Mathematics) > Unit 2

Lesson 3: Lesson 7: Similar polygons# Dilations and properties

CCSS.Math:

What properties of a shape are preserved or not preserved after a dilation?

## Want to join the conversation?

- Where did you get this dilate tool?(29 votes)
- It was in Khan, but not anymore. Here is one not on Khan: https://www.desmos.com/calculator/61qfatzci1(20 votes)

- At2:16, what does "prime" mean?(7 votes)
- The word "prime" can mean several things in mathematics. Most commonly, it's either "prime number" or the "prime symbol". Here, the second meaning is used: the prime symbol.

The prime symbol looks like a single quote: '

It is usually used after a letter, like: x' is "x prime". Its general meaning is "a modified version of the preceding letter". So x' is "a modified version of x", or sometimes "a value related to x".

In the video, Sal uses "B prime" or B' to mean the vertex (= point) that corresponds to B, but on the other (blue) figure.(35 votes)

- Does anyone use khan anymore? most of these say 2-3 years ago(7 votes)
- I use it sometimes and I am in march 2020 right now!

(well, when some of you view this it may not be march 2020)(16 votes)

- What does it mean for a line to be preserved?(6 votes)
- Preserved means "stay the same over time",

In this video on dilation, it means: does the line stays the same after the dilation. If the new line sit exactly on top of the first line, then yes.

For a line segment to be preserved, the new line segment needs to sit exactly on top of the first one, which also means they need to start at the exact same point and end at the exact same point.

Hope that helps!(6 votes)

- Do dilations preserve perimeter?(5 votes)
- No, the length of any curve, straight or not, is multiplied by the scale factor of the dilation. So if your figure has perimeter, 6, then after a dilation with a scale factor of 1/3, it will have a perimeter of 2, regardless of the original shape.(5 votes)

- How do you know if they are on parallel lines?(6 votes)
- the lines are next to each other... like:

| | or \ \ or / /

they**NEVER**Intersect.

You know if they are on a parallel line if you visibly see it on the problem.

I hope this helped!(2 votes)

- don't the proportion between the lines like the proportion between line segment AB and CB stay the same in a-prime b-prime and c-prime b-prime(2 votes)
- Yes they do. When you dilate a figure you are just multiplying and/or dividing the proportion.(8 votes)

- What does distinct mean?(3 votes)
- Distinct means not the same. If I say 'pick two distinct points in the plane', that means you're not allowed to pick the same point twice.(5 votes)

- At4:48he tells that the angles are preserved. Is there a proof? or is it just an axiom?(3 votes)
- Draw a triangle in the plane, and then draw its image after a dilation. Because the straight sides of the triangle are all scaled evenly, the triangles are similar, and so all of their corresponding angles are equal.(5 votes)

- i really needed help

with mapping shaps(4 votes)- Watch the video over and over. Or just try the practice questions and focus on the concepts and not the answers (Memorizing which one is right or wrong).(2 votes)

## Video transcript

- [Instructor] What we're
going to do in this video is think about how shapes' properties might be preserved or not
preserved from dilations and so here we have this quadrilateral and we're going to dilate
it about point P here and I have this little Dilation tool. So the first question is are the coordinates of the
vertices going to be preserved? Well, pause the video and
try to think about that. Let's just try it out experimentally. We can see under an
arbitrary dilation here, the coordinates are not preserved. The point that corresponds to D now has a different coordinate. The vertices, the vertex
that corresponds to A now has different coordinates. Same thing for B and C. The corresponding points
after the dilation now sit on a different part
of the coordinate plane. So in this case, the
coordinates of the vertices are not preserved. Now, the next question, let
me go back to where we were. So the next question, the corresponding line
segments after dilation, are they sitting on the same line and so let me dilate again and so you can see if you
consider this point B prime 'cause it corresponds to point B, the segment B prime C prime, this does not sit on the same line as BC but the segment D prime, the corresponding line
segment to line segment AD, that does sit on the same line and if you think about why that is, well, if we originally draw a line that, if we look at the line
that contains segment AD, it also goes through point
P and so as we expand out, this segment right over here is going to expand and shift
outward along the same lines but that's not going to be
true of these other segments because they don't, because the point P
does not sit on the line that those segments sit on and so let's just expand it again so you see that right over there. Now, the next question, are
angle measures preserved? Well, it looks like they are and this is one of the things that is true about a dilation is that you're going to
preserve angle measures. This angle is still a right angle. This angle here, I guess
you can call it angle, the measure of angle B is the same as the
measure of angle B prime and you can see it with all of
these points right over there and then the last question. Are side lengths, perimeter
and area preserved? Well, we can immediately
see as we dilate outwards, for example, the segment
corresponding to AD has gotten longer. In fact, if we dilate
outwards, all of the segments, the corresponding segments
are getting larger and if they're all getting larger then the perimeter's getting larger and the area's getting larger. Likewise, if we dilate in like this, they're all getting smaller. So side lengths, perimeter
and area are not preserved. Now, let's ask the same
questions with another dilation and this is going to be interesting because we're going to look at a dilation that is centered at one of
the vertices of our shape. So let me scroll down here and so I have the same tool again and now here we have a
triangle, triangle ABC and we're gonna dilate about point C. So first of all, do we think the vertices, the coordinates of the vertices
are going to be preserved? Let's dilate out. Well, you can see point C is preserved. When it gets mapped after the dilation, it sits in the exact same place but the things that correspond
to A and B are not preserved. You could call this A prime and this definitely has
different coordinates than A and B prime definitely has
different coordinates than B. Now, what about
corresponding line segments? Are they on the same line? Well, some of them are
and some of them aren't. So for example, when we dilate, so let's look at the segment
AC and the segment BC, when we dilate, we can see, whoops, when we dilate, we can see
the corresponding segments, you could call this A prime
C prime or B prime C prime, do still sit on that same line and that's because the point
that we are dilating about, point C, sat on those original segments. So we're essentially just lengthening out on the point that is not
the center of dilation. We're lengthening out away from it or if the dilation is going in, we would be shortening
along that same line but some of the segments are not overlapping on the same line. So for example, A prime B prime does not sit along the same line as AB. Now, what about the angle measures? Well, we already talked about it. Angle measures are
preserved under dilations. The measure of angle C here,
this is the exact same angle and so is the measure of angle
you could call this A prime and B prime right over here. And then finally, what about side lengths? Well, you can clearly see
that when I dilate out, my side lengths increase
or if I dilate in, my side lengths decrease and so side lengths are not preserved and if side lengths are not preserved then the perimeter is not preserved and also the area is not preserved. You could view area as a
function of the side lengths. As we dilate out like this, the perimeter grows and so does the area. If we dilate in like this, the perimeter shrinks
and so does the area.