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

Lesson 3: Lesson 7: Similar polygons

# Dilations and properties

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?
• At , what does "prime" mean?
• 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.
• Does anyone use khan anymore? most of these say 2-3 years ago
• 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)
• What does it mean for a line to be preserved?
• 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!
• Do dilations preserve perimeter?
• 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.
• How do you know if they are on parallel lines?
• 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!
• 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
• Yes they do. When you dilate a figure you are just multiplying and/or dividing the proportion.
• What does distinct mean?
• 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.