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## High school geometry

### Course: High school geometry > Unit 1

Lesson 6: Dilations# Dilating triangles: find the error

CCSS.Math:

Dilating doesn't just shrink or grow a figure. The scale factor also changes the distance from each point to the center of dilation. Created by Sal Khan.

## Want to join the conversation?

- So at 0.48, shouldn't it be 1/2, rather than 3/2? Forgive me if I am wrong.(3 votes)
- yes, I believe it should be 1/2(7 votes)

- At the end of the video, sal said that only choice C is correct in this question! but what happen to "
*select all that apply*"? wouldn't choice A be correct too? (they have the same scale factor) Why is choice A incorrect?(4 votes)- While the triangle in A does appear to have been rescaled by the same factor, the distances between each point and P have not been. To dilate a shape is to multiply the distance from each point to the center of dilation by the scale factor, which often means that the figure moves, and doesn't simply resize.(4 votes)

- i hate khan so much(4 votes)
- Please tell Sal to make a specific video for finding(guessing) the center of dilation, as well an exercise on it.(3 votes)
- would the answer be c?(2 votes)
- wait did he not say last video that you need to center the triangle to the center of p(2 votes)
- Are there more accurate ways to calculate dilations? Using angles and lengths and their precise measurements?(2 votes)
- Can we exactly (mathematically) calculate which ones are correct? Why or why not?(1 vote)
- We can't calculate it very exactly, because we're given pictures and not exact information. But we can pick out features of the pictures and use them to find contradictions like Sal did.(2 votes)

- what is 69 times 53?(0 votes)

## Video transcript

- [Instructor] We are told triangle A-prime, B-prime, C-prime is the image of triangle ABC under a dilation whose center is P and scale factor is 3/4. Which figure correctly shows triangle A-prime, B-prime, C-prime
using the solid line. So pause this video and see if you can figure this out on your own. All right, now before I
even look at the choices, I like to just think about, what would that dilation
actually look like? So our center of dilation is P. And it's a scale factor of 3/4. So one way to think about it is, however far any point was from P before, is now going to be 3/4 as
far, but along the same line. So I'm just going to estimate it. So if C was there, 3/2 would be this far. So 3/4 would be right about there. So C-prime should be about there. If we have this line
connecting B and P like this, let's see, half of that is there. 3/4 is going to be there. So B-prime should be there. And then on this line,
halfway is roughly there. I'm just eyeballing it. So 3/4 is there. So A-prime, A-prime, should be there. And so A-prime, B-prime, C-prime should look something like this. Which we can see is exactly
what we see for choice C. So choice C, it looks correct. So I'm gonna just circle that, or select it just like that. But let's just make sure we understand why these other two choices were not correct. So choice A, it looks
like it is a dilation with a 3/4 scale factor. Each of the dimensions, each of the sides of these triangles, of this triangle, looks like it's about 3/4
of what it originally was. But it doesn't look like
the center of dilation is P. Here the center of dilation looks like it is probably the
midpoint of segment AC. Because now it looks
like everything is 3/4 of the distance it was to that point. So they have this other center
of dilation in choice A. The center of dilation is not P, and that's why we can rule that one out. And then for choice B right over here, it looks like they just
got the scale factor wrong. Actually they got the center of dilation and the scale factor wrong. It still looks like they are using this as a center of dilation. But this scale factor
looks like it's much closer to 1/4 or 1/3, not 3/4. So that's why we can rule
that one out as well. We like our choice, C.