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### Course: 4th grade > Unit 11

Lesson 2: Angle introduction# Drawing acute, right and obtuse angles

Learn how to draw acute, right, and obtuse angles with given points. Created by Sal Khan.

## Want to join the conversation?

- How many times in life do you think we use angles?(49 votes)
- Us humans use angles lots and lots of times in our times. For example, if you have a job, that has to do with building or creating, (a engineer) you would use angles almost every day of your life. You need to know how much degrees to build a corner or wall. Angles is a really important part to learn in our lives.(17 votes)

- i wonder if there is fraction angles like: 5 and 2 over 7 or something like that(26 votes)
- Radians are often measured with fractions. For example: pi / 2 = 90°(1 vote)

- why do we have to know angles(9 votes)
- in order to define and study polygons such quadrilaterals and use in our everyday life.(6 votes)

- So Obtuse Is more than 90, Acute is less than 90 and right is 90? I'm Not Sure about The Right One, Please tell me!(8 votes)
- Yeah, obtuse is is more than 90, acute is less than 90, right is 90, straight is 180, and reflex is more than 180.(6 votes)

- What are the exact degree's for all angles?(7 votes)
- acute angles are between 0 and 90, right angles are 90, obtuse angles are between 90 and 180, straight angles[straight line] are 180, reflex angles are between 180 and 360, and complete angles are 360 degrees.(1 vote)

- How many types of angles are there?(4 votes)
- 4:acute,obtuse,right angle and reflex.A acute angle is less than 90 degrees,a right angles is 90 degrees,a obtuse angle is 90-180 degrees and a reflex is more than 180 degrees.(3 votes)

- what is a acute angle again(3 votes)
- an angle less than 90degrees

you can use this trick to remember

a"cute": cute things are normally small

A"cute" angle is small or lesser than 90 degrees.

:D

i hope this helped

^U^(2 votes)

- what is the difference between straight angle and complete angle?(2 votes)
- Straight angle creates a line while a complete angle only creates a single ray. Or in angles 180° is straight and 360° complete.

Complete is basically like a 0° angle, in the sense there won't be a gap, between the two lines.

Hope this helped.(4 votes)

- Isn't an obtuse angle more than 90 degrees but less than 180 degrees?(4 votes)
- no a straight angle is 180 degrees and it would still be a obtuse angle though(0 votes)

- Hi. I use Khan Academy with my class and it is great--thank you. We have though had some problems with several of the exercises in this unit that require the students to draw angles virtually. It would be great if the drawing tools had "snap to" such that it "catches" when a point is close to a target point. Right now it doesn't, and students get frustrated trying to get the points exactly on top. Also, I have noticed questions regularly being marked incorrect that are not--it seems some problems require a specific ray be dropped on the target point. If you switch the rays you still have an angle that meets the requirements, but it is marked wrong. Thanks for your time.(3 votes)

## Video transcript

Put the vertex of the angle
at point A. So let me do that. Make one of the rays
go through point B. Make the other ray go
through one of the other points to make an acute angle. So an acute angle is an
angle less than 90 degrees. So we could make one of these
rays go through point B. And then we have to pick where
to put the other ray to make it go to one of
the other points. And we have to be
very careful here because we have to look at
this arc that shows which angle the tool is
actually measuring. Because we might be tempted
to do something like this, thinking that, hey,
maybe this is the angle that we're thinking about. But the tool thinks we're
referring to this outer angle right over here, this
larger huge angle. This angle right over here
is well over 180 degrees. So we have to pay
attention to this arc to make sure that the tool
is looking at the same angle that we are. So once again, we
want an acute angle. So this right over here
looks like an acute angle. It looks like it is
less than 90 degrees. And we have to be very
careful that we go exactly through that point. So that looks about right. This is an acute angle
because its measure is less than 90 degrees. Let's do a few more of these. Make an obtuse angle
using the black points. Choose one of the
points as the vertex and make the rays go through
the other two points. The angle should also be
less than 180 degrees. So you could think
of it several ways. You could just try
to pick that point. But then when you go through
these other two points, this right over here
is an acute angle. This is less than 90 degrees. You could do something
like this where now when I switch the rays, the tool is
now thinking about this angle, not this outer angle
right over here. But this is larger
than 180 degrees. So this also doesn't apply. So we really picked the
wrong point for the vertex. If you just move this out
of the way and eyeball it, it looks like you get
the largest angle that's less than 180 degrees if you
put the vertex right over here. So let me do that. Put the vertex right over there. And now it looks like I have
constructed a 180-degree angle. And we have to be
very careful that we go right through those
points because otherwise it might mark us wrong. So that looks pretty good. That's an obtuse angle
because its measure is greater than 90 degrees. Let's do one more of these. So another obtuse
angle using the points. Same general idea so that would
be an exactly 180-degree angle. And there we go. And this is an obtuse
angle because it is greater than 90 degrees.