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## 4th grade

### Unit 12: Lesson 2

Measuring angles# Constructing angles

CCSS.Math:

Sal uses a protractor to create 10° and 155° angles. Created by Sal Khan.

## Video transcript

We're asked to construct
a 10 degree angle. So we have this
little angle tool here that we can use
to construct an angle. So just like that. And then they give
us a protractor to actually measure the angle. So let's set it up. Let's put the protractor here. Let's put the
vertex of the angle at the center of the protractor,
in other words, center the protractor at the
center of the vertex. Looks like the protractor's
a little bit easier to manipulate. So let's do that there. Now let's put one of the
rays here at 0 degrees. And now I'm going to put
the other ray at 10 degrees. And it looks like I am done. I have constructed
a 10 degree angle. And you want to be
careful here when you use this tool because
the angle in question-- let me move the protractor show
you what I'm talking about-- that this right over here is the
angle that we're talking about. If I'd switched these two rays,
the way the tool is set up, it might have
interpreted the angle as this outer angle
right over here. So be careful to look at which
angle we're actually measuring. But this looks like I
did the 10 degree angle. So let me check my answer. And just to be clear
what I'm talking about. If you did the 10
degree angle like this, then it definitely would
have marked it wrong even though the interior angle
right over here or this angle right here might be 10 degrees. The way the tool is set up,
you see from this circle, that it thinks that you're
looking at this outer angle. So it's important to
make sure, at least for the sake of this
exercise, that the system, that the computer
program knows which angle you're going
to talk about. Let's do one more of these. 155 degree angle. And this one's
interesting because this is an obtuse angle. So once again, let
us put the protractor at the vertex of the angle. So just like that. And now that seems pretty good. Now let's take one ray
and put it at 0 degrees, and then let's take the
other one and put it at 155. So once again, 10, 20,
30, 40, 50, 60, 70, 80, 90-- that gets us
to a right angle. Then we'll start getting into
obtuse angles, 100, 110, 120, 130, 140, 150. And just to make
sure that blue arc is measuring this angle right
over here, not the outer one. And let me move the
protractor out of the way so we can get a good look at it. And we got it wrong. So let's see what we--
oh, 155 degree angle, not 150 degree angle. Let me fix that. 155 degree angle. Now we got it right.