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### Course: 4th grade (Eureka Math/EngageNY) > Unit 4

Lesson 4: Topic D: Two-dimensional figures and symmetry- Intro to reflective symmetry
- Identifying symmetrical figures
- Identify line symmetry
- Symmetry review
- Classifying triangles by angles
- Classifying triangles
- Worked example: Classifying triangles
- Classify triangles by angles
- Classify triangles by side lengths
- Types of triangles review
- Classifying shapes by line and angles types
- Quadrilateral properties
- Classifying shapes by lines and angles
- Classify shapes by line and angle types
- Polygons review
- Classify triangles by both sides and angles

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# Classifying shapes by line and angles types

Sal categorizes shapes based on their sides and angles. Created by Sal Khan.

## Want to join the conversation?

- What do those two extra tiny lines mean on the triangle at1:05?(40 votes)
- This simply means that the two sides of the triangle are equal in length.(8 votes)

- Most of the triangles have multiple types of angles in them. How can you tell the name of the triangle if it has multiple degree angles?(5 votes)
- If one angle is a right angle than it's a right triangle, u don't have to look at the other angles. Same for obtuse angles. But for acute triangles all of the angles have to be acute for it to be an acute triangle.(3 votes)

- I know perpendicular means intersecting at 90*, and parallel means the two lines never intersect. What about the lines that intersect at more or less than 90*? Is there something that would explain that to me?(3 votes)
- keep in mind that if two lines intersect, and it is not a right angle, you will have a pair of acute angles and a pair of obtuse angles. Two lines intersecting will create 4 total angles(5 votes)

- Is that firstone a trapezoid? If so is it a right trapezoid?(5 votes)
- If a line is perpendicular it make a right angle?(4 votes)
- Perpendicular lines intersect at a 90 degree angle, so yes.(2 votes)

- could someone help me with this?(4 votes)
- how do i know if an angle is greater or lesser than a right angle(2 votes)
- A few extra.
**acute**less then 90°**right**exactly 90°**obtuse**between 90° and 180°**straight**exactly 180°

And**reflex**between 180° and 360°(3 votes)

- what is a scalene triangle?(2 votes)
- a scalene is triangle where all 3 side are different, you can memorize this by thinking about how scales are never the same because they change so much(3 votes)

- hey can you get me out of class(2 votes)

## Video transcript

Which side is
perpendicular to side BC? So BC is this line
segment right over here. And for another segment
to be perpendicular to it, perpendicular just means
that the two segments need to intersect at a right
angle, or at a 90-degree angle. And we see that BC intersects
AB at a 90-degree angle. This symbol right over here
represents a 90-degree, or a right angle. So we just have to
find side AB or BA. And that's right over here. Side AB is perpendicular
to side BC. Let's do a few more of these. Put the triangles into
the correct categories, so this right over here. So let's see. Let's think about
our categories. Right triangles-- so that means
it has a 90-degree angle in it. Obtuse triangles-- that
means it has an angle larger than 90 degrees in it. Acute triangles-- that
means all three angles are less than 90 degrees. So this one has a
90-degree angle. It has a right angle
right over here. So this is a right triangle. This one right over
here, all of these angles are less than 90 degrees,
just eyeballing it. So this is going to
be an acute-- that's going to be an acute triangle. I'll put it under acute
triangles right over there. Then this one over here,
this angle up here, this is-- and we can
assume that these actually are drawn to scale, this is more
open than a 90-degree angle. This is an obtuse
angle right over here. It's going to be
more than 90 degrees. So this is an obtuse triangle. Now, this one over here,
all of them seem acute. None of them even seem
to be a right angle. So I would put this again
into acute-- acute triangles. This one here clearly
has a right angle. It's labeled as such. So we'll throw it
right over here. And then this one, this
angle right over here is clearly even larger. It has a larger measure
than a right angle. So this angle right over
here is more than 90 degrees. It's going to be
an obtuse angle. So we will throw it into
obtuse-- obtuse triangles. So we got two in each of these. And let's check our answer. We got it right.