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## 4th grade

### Unit 12: Lesson 2

Measuring angles# Measuring angles using a protractor

CCSS.Math:

Learn to use a virtual protractor to measure angles. Created by Sal Khan.

## Video transcript

What I have right in front
of me is the Khan Academy measuring angles exercise. I have a small part of it in
this screen right over here. And it's a pretty cool
exercise because it has this little
virtual protractor that we can use to
actually measure angles. And I want to give
credit to the person who built this protractor, because
I think it's pretty neat. Omar Rizwan, who was actually
a high school intern, made this pretty neat module. And so in general, when you
want to measure an angle, what you want to do
is you want to put the center of your
protractor at the center of-- or I should say, at the
vertex of the angle. Or you want to put the
vertex of the angle at the center of the protractor. And then what you want to do
is either rotate the angle or rotate the protractor. In this case, we're going
to rotate the protractor. You want to rotate
the protractor so that the 0 angle,
or kind of the 0 mark, is at one of the
sides of the angle. And the other side of the
angle is within the protractor. So let's try to do that. So maybe if we want to
do that, this 0 side should be at this
side of the angle. So let's rotate it that way. Let me just keep rotating it. If I could just keep it pressed. That's better. All right. That looks about right. So one side is at the 0 mark. And then my angle,
my other side-- or if this was a
ray, it points to, looks like, pretty close
to the 20 degree mark. So I will type that
in off the screen. You don't see that. And that is the right answer. And then we can
get another angle. So let's try to measure
this one right over here. So once again, place the
center of the protractor at the center, at the
vertex, of our angle. We can place the 0 degree,
the base of the protractor, at this side of the angle. So let's just rotate it a
little bit, maybe one more time. That looks about right. And then the angle is now
opening up-- let's see, the other side is
pointing to 110 degrees. So this is larger
than 90 degrees. It's also an obtuse angle. The last one was an acute angle. This is obtuse, 110 degrees. More than 90 degrees. So let me type it in. I got the right answer. Let's do a couple more of these. So once again, put the
center of the protractor at the vertex of our angle. And now, I want to rotate it. There we go. And this looks like roughly
an 80 degree angle, not quite. If I have to be
really precise, it looks like it's maybe
81 or 82 degrees. But I'll just go with
80 as my best guess. I got the right answer. Let's do one more of these. So once again,
vertex of my angle at the center of my protractor. And then I want to put one side
of the angle at the 0 degree. And I want to show you,
there's two ways to do that. You could do this. You could do just this. But this isn't too helpful,
because the angle is now outside. The other side sits
outside of the protractor. So you want the 0
degrees on the side, so the other side is
within the protractor. So let's keep rotating it. There we go. And then our other
side opens up or you could say points to 70 degrees. So this is an acute
angle right over here. So it is 70 degrees. So I'll leave you with that. Oh look, I'm ready to move
on, the exercise tells me. And now we can start talking
about more things about angles now that we know
how to measure them.